Reading a Phase Shift

To read y = a sin(x − h), with a positive, off a graph, find the amplitude from the top, then find a place where the wave crosses its middle going up: that x is h. A wave has one such crossing every period, so pick the first one at or to the right of 0 and the answer is unique.

This wave tops out at 2 and crosses going up at 90°, so it is $y = 2\sin\left(x-90^{\circ}\right)$. The same curve is −2 cos x, but the form asked for decides how you write it.