Well you should be familiar with the shape of y=cos(x) in the Cartesian plane (you could also think in terms of triangles and the unit circle), and know that
cos(0) = 1
cos(pi/2) = cos(-pi/2) = 0
cos(pi) = cos(-pi) = -1
cos(2pi/3) = cos(-2pi/3) = 0
cos(2pi) = cos(-2pi) = 1
etc.
So, your answer is correct, and another answer is given by
5x = pi/2
x = pi/10