please help geometry

$r^2 + 18^2 = (r+6)^2$

$r^2 + 324 = r^2 + 12r + 36$

$12r = 288$

$\boxed{r = 24}$

POQ  forms  a right triangle with  angle QPO  = 90 degrees     (a tangent always meets a radius at 90° )

So....using the Pythagorean Theorem

OP^2  = QO^2  -  PQ^2

OP^2   = (r + 6)^2 -  18^2

r^2  = r^2 + 12r  + 36  - 324

12r   =  324  - 36

12r  =  288

r  = 288  / 12     =   24