From a point P on the ground the angles of elevation of the top of the building and of a helicopter, hovering at some height over the top of the building, are 30 degree and 60 degree, respectively. Find the height of the helicopter above the ground

Answer 1

draw a diagram.
If the building has height h, and
the chopper is x above the building

h cot60° = (h+x)cot30°

Both of these are equal to the distance of P from the building. Once you have that, you can find h and x.

Without knowing that or h, you cannot find x.