Getting the third point from two points on one line

Assume: $$dx = x2 - x1$$ $$dy = y2 - y1$$

---

Then, $$x3 = x1 + dx*k$$ $$y3 = y1 + dy*k$$

---

The square of distance between $p1$ and $p3$ is:

$$(dx^2 + dy^2)*k^2 = 300^2$$

Now you can find $k$ and then $x3$ and $y3$