Formulae for points given other positions

Imagine a line AB crossing the x-axis at B (xb,0) and y-axis at A (0,ya).
Now imagine a shorter PQ, length 2L, at right angles to the line AB, and such that the length is L on both sides of the line AB. xp, yp, xq and yq are all positive.

What are the formulae for xp, yp, xq and yq?

I hope someone can help.

John