Question

sin(2x)=-1 how do you solve it? I already know that answer.

Answer 1

do you know how to solve sin(u)=-1?

Answer 2

solve sin(u)=-1
for u
look for a u on your unit circle satisfying sin(u)=-1

Answer 3

http://upload.wikimedia.org/wikipedia/commons/4/4c/Unit_circle_angles_color.svg
Here is a unit circle you need it.
(cos is the first value, sin is the second value)

Answer 4

sin 2x = -1 = sin 3Pi/2
2x = 3Pi/2 -> x = 3Pi/4