A large well labeled diagram is **essential**. At the risk of a 12 pt purple coloured attack, I must ask:

Have you drawn one?

Let the observers be at A and B. Let the helicopter be at C. Let the perpendicular line from C meet AB at D.

Let AD = x so that DB = 100 - x. Let DC = h = height of helicopter.

From right-triangle ADC:

.... (1)

From right-triangle BDC:

.... (2)

Equate equations (1) and (2) and solve for x (your value for x should be more accurate than the accuracy required for the value of h). Substitute the value of x into equation (1) and solve for h.