Hello jashansinghalLook at the diagram I have attached.

Because two of the angles are equal ( ) the point in the plane, , lies on the diagonal produced of the square base of the tower.

Therefore, in the diagram, .

Also,

Therefore, if the height and the breadth of the tower are and respectively:and

and

Therefore, using the Cosine Rule on :

Multiply both sides by :

Treating this as a quadratic in , and using the formula:

Grandad, since the other root is negative