As I noted in another thread, Sin(x) and sin(x) are technically two different functions. The common trig functions are written without capital letters.

sin(x) = -1

We know that, for 0 <= x < 2(pi), that sin(x) = -1 for only x = 3(pi)/2. This solution is periodic with a period of 2(pi), so the solution will be:

x = 3(pi)/2 + 2(pi)*n, where n is any integer.

-Dan