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.