sin(x-y) = sin(x)cos(y) - cos(x)sin(y)
I'm pretty sure that this should help matters a bit. If you need a bigger hint let me know.
The height, h, in metres above the ground of a rider on a Ferris wheel can be modelled by the equation h=10sin(pi/15t-7.5)+12, where t is the time in seconds. At t=0 s, the rider is at the lowest point. Determine the first two times that the rider is 20 m above the ground. Also use a graph to illustrate your solutions.