7th class question my daughter asked and need answer if possible

There are slicker ways, but the question can fairly easily be solved by brute force. A good lesson for her to learn is that it rarely hurts to experiment a bit with the data and look for patterns. Let’s take a look at what happens over the first two weeks or so:

$$\begin{array}{c} \text{Su}&\text{Mo}&\text{Tu}&\text{We}&\text{Th}&\text{Fr}&\text{Sa}&\text{Su}&\text{Mo}&\text{Tu}&\text{We}&\text{Th}&\text{Fr}&\text{Sa}&\text{Su}&\text{Mo}\\ \text{R}&\text{W}&\text{W}&\text{W}&\text{W}&\text{R}&\text{W}&\text{W}&\text{W}&\text{W}&\text{R}&\text{W}&\text{W}&\text{W}&\text{W}&\text{R} \end{array}$$

Here $\text{R}$ is a rest day, and $\text{W}$ is a work day. Look at the sequence of rest days: Sunday, Friday, Wednesday, Monday. Clearly they are moving backwards two days in the week for each cycle of five days. To get to Sunday again, they have to continue to move back through Saturday, Thursday, and Tuesday, after which the next cycle has its rest day on Sunday again. That’s seven cycles altogether, a total of $7\cdot5=35$ days. His working Monday is the first of those $35$ days, so it will be another $34$ days until he has his next Sunday rest day.

A slightly more sophisticated approach is to realize that to get a rest day on Sunday, he must go through a whole number of $5$-day work cycles and a whole number of weeks. In other words, the number of days from one Sunday rest day to the next must be both a multiple of $5$ and a multiple of $7$. The least common multiple of $5$ and $7$ is $35$, so Sunday rest days must fall $35$ days apart, and the next one will therefore fall $34$ days after the Monday start.

The idea is that the cycle of $5$ days of work/rest, and the weekly cycle of $7$ days will complete at the same time in a number of days equal to the least common multiple of $5$ and $7$. In this case this is simply the product $5 \times 7 = 35$. (In general, the least common multiple of two numbers equals their product, divided by their greatest common divisor.) So after $34$ days the rest day will fall again on a Sunday.

You can help by providing your daughter with an old calendar, and have her mark off the rest days explicitly, until she first bumps into Sunday. That is a perfectly legitimate solution. If the work needs to be handed in, the calendar with the marked off rest days is a complete mathematical proof. Then one might want to explore the markings to see whether there is structure there that makes it possible to reach the answer more quickly. The point of the above approach is that your student will be in concrete control every step of the way, she will know precisely what's going on.

Perhaps better, from the point of view of learning, is for the student to produce her own calendar. It may be a good idea to label the days $1$, $2$, $3$, $\dots$, $30$, $31$, $32$, and so on, so that numerical patterns in the marked off days can be more readily detected. But this is certainly not necessary.

The following is a much more abstract version, which should only be done after the concrete manipulation with the calendar. Think of the first Sunday as Day $1$. Then the next Sunday is Day $8$, the one after that is Day $15$, then $22$, $29$, $36$, $43$, and so on.

Now make a list of the rest days. The first one, we are told, was on Day $1$. The next one is Day $6$, then Day $11$, then $16$, then $21$, and so on. Continue until we bump into a Sunday. It will be soon: if we continue the rest days, we get $26$, $31$, $36$, got it!