1.
we can first start off by figuring out how many degrees are in each interior angle of a hexagon: (6-2)*180$^\circ$=720$^\circ$ $\frac{720^\circ}{6}=120^\circ$ and we know that the triangle with B and C as the two base vertices is an isoscolese, because the square and the hexagon have equal side lengths, so angle B = angle C. so we have
$\frac{180-(120-90)}/2$= angle C = $75^\circ$
and because we want to find angle ABC, we have to subtract 75$^\circ$ from 120 to get angle ABC = 120-75 = $\boxed{45^\circ}$
2.
Because of power of point, we know that PC$\cdot$CD = PA$\cdot$AB = 25 = 4AB
so AB=$\boxed{\frac{25}4}$
3.
The diagonal of rectangle ABCD is the diameter of the circle, so
4$^2$+6$^2$= CD^2 = 52
CD= 2$\sqrt{13}$
so the area of the square is:
$(\frac{2\sqrt{13}}2)^2\cdot\pi=13\pi$
so
shaded region = $\boxed{13\pi-24}$
.