How to find a basis for $2\times 2$ matrix

An arbitrary element of $W$ is $$ A=\begin{pmatrix} a & a \\a & b\end{pmatrix}=a\begin{pmatrix} 1 & 1 \\1 & 0\end{pmatrix}+b\begin{pmatrix} 0 & 0 \\0 & 1\end{pmatrix} $$ So a basis of $W$ is $\{\begin{pmatrix} 1 & 1 \\1 & 0\end{pmatrix}, \begin{pmatrix} 0 & 0 \\0 & 1\end{pmatrix}\}$

Hint: Every $2\times 2$ matrix may be written as $$ \begin{bmatrix} a & b\\ c& d \end{bmatrix} = a\begin{bmatrix} 1 & 0\\ 0 & 0 \end{bmatrix} + b\begin{bmatrix} 0 & 1\\ 0 & 0 \end{bmatrix} + c\begin{bmatrix} 0 & 0\\ 1 & 0 \end{bmatrix} +d\begin{bmatrix} 0 & 0\\ 0 & 1 \end{bmatrix} $$ This shows that $$ \left\{ \begin{bmatrix} 1 & 0\\ 0 & 0 \end{bmatrix} ,\begin{bmatrix} 0 & 1\\ 0 & 0 \end{bmatrix} ,\begin{bmatrix} 0 & 0\\ 1 & 0 \end{bmatrix} ,\begin{bmatrix} 0 & 0\\ 0 & 1 \end{bmatrix} \right\} $$ is a basis for the vector space $M_2$ of all $2\times 2$ matrices.

Can you construct a similar argument for your subspace $W$?

You've already given us a parametrization in two variables, $a$ and $b$. What you've essentially told us is that you are looking for matrices of the form

$$\begin{pmatrix} a&a \\ a&b\end{pmatrix} = a\begin{pmatrix} 1&1 \\ 1&0\end{pmatrix} + b\begin{pmatrix} 0&0\\0&1\end{pmatrix},$$

so that $\begin{pmatrix} 1&1\\1&0\end{pmatrix}$ and $\begin{pmatrix} 0&0\\0&1\end{pmatrix}$ form a basis for your space. From this, I surmise that this is a question coming immediately after you have covered bases in a linear algebra class or text (which is fine). Does this make sense? It is important to understand bases in a linear algebra class - they form the basis for many things.