Let XY be a tangent to a circle, and let XA be a secant of the circle, as shown below. If AX = 15 and XY = 9, then what is AB?
Sorry, I forgot to post my diagram -
By power of a point, AB = 29/3.
There is a simple ratio formula fo this. I have seen Heureka use it. I've forgetton what it is though.
It is the middle one
In your case
9^2=XB * 15
81 = XB *15
XB = 81/15 = 5.4 units