What is the length of PQ?
Draw the altitude line from the center of the circle down to side AB, bisecting PQ at a right angle. What is the length from the bisection point to P?
Draw the radius line from the center of the circle to side BC. Label this as "r". Note that, by nature of tangency, this radius must be perpendicular to BC.
Using BP and the length found above, what is the distance from B to the bisection point?
How does this relate to the radius?