8 sin(x/2) - 5 = 0 for 0 < x < 2pi
8 sin(x/2) = 5
sin(x/2) = 5/8
x/2 = sin-1(5/8)
x = 2·sin-1(5/8)
Use you calculator to get the answer. Be sure it's set to radians, not degrees. There are actually two answers on the interval 0<x<2pi, one in the first quadrant (which your calculator gives you) and one in the second quadrant at pi-x radians.