Didn't I tell you before to memorize certain trigonometric values? It seems you have not fulfilled this assignment.

sin(-3pi/4) = -sin(3pi/4) = -sin(pi/4) = ???

cos(5pi/6) = -cos(pi/6) = ???

Now what?

Note: If you're using LaTeX, feel free to put a slash in front of those "pi"s. Excellent results can be obtained.