Solve
sin 2 theta = 0.8

In this question it is actually necessary to find the value of theta. there are several methods by which the value of theta can be determined. One of the procedures is described below. As per my understanding, this is the easiest procedure.
sin2 theta = 0.8
Dividing both sides by sin we get the equation in a new format.
2 theta = sin⁻¹ 0.8
2 theta = 53.13
theta = 53.13/2
         = 26.565
         = 26.6
So the value of theta is 26.6 degree. Hopefully this will make the problem clear to you and it is the easiest method as per my judgement.