As far as I understand your sketch ...
1. The vertex of the pyramid (Obviously you labeled it V) is perpendicularly above the intersection M of the two diagonals of the base area.
The diagonal has a length of 25.
2. The height h, a half diagonal and an edge form a right triangle:
Solve for h.
3. The angle is half as large as the angle . Use the right traingle MCV to determine the angle :