First, bring -25 to the other side: +25 which gives
x3 = 128 - x3
Now, same with - x3 ( + x3 for both sides), giving
2x3 = 128
Use substitution u=cos(x) resulting in du=-sin(x)dx: ∫0π/2sin(x)cos(x)^2dx = ∫0π/2-u^2du = [-1/3 u^3]x=0x=π/2 = [-1/3 cos(x)^3]0