What have you tried?
Do you know the 'intersecting secant theorem'?
Intersecting Secant Theorem - Math Open Reference
Along the theorem:
But we can write and therefore:
You have given and so you can substitute them in (1). You have also given:
If we susbtitute (1) in (2) we get:
If you have substituted the given values for PC and PA into this equation then you will have a quadratic equation in one variable AB which you can solve (you will get two solutions but offcourse you have to reject the negative one because the lenght of a side can't be negative).
If you want a proof then you can take a look here: