Getting the third point from two points on one line

Question

My question is the following

How can I get point $(x_3, y_3)$ from points $(x_1, y_1)$ and $(x_2, y_2)$ ?

The distance of point $(x_3, y_3)$ from $(x_1, y_1)$ is $300$.

Answer 1

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

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

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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 $y3$