Some Help Please

Suppose that $\begin{array}{@{}c@{\;}c@{}c@{}c} && &P_b \\ & \times & & P_b \\ & & Q & P_b, \\ \end{array}$ where P, Q, and b represent three distinct digits 1-9. If$Q=\frac{P}{2}$, and P is two less than b, what is the value of the digit P?

(b means base b)

Sorry about the latex, don't know how to do a horizontal line. ( Please help)