2. As shown in Figure 1, given that $P A$ and $P B$ are tangent to $\odot O$ at points $A$ and $B$, $P C$ satisfies $A B \cdot P B - A C \cdot P C = A B \cdot P C - A C \cdot P B$, and $A P \perp P C$, $\angle P A B = 2 \angle B P C$. Then $\angle A C B =$ $\qquad$