I will try my usual rule of not giving a direct answer but this case might be hard.
I assume degrees rather than radians (not that it matters at this stage but I have seen such things end up in calculus territory).
First stage: rearrange. I went for nothing as a denominator.
Second stage: trigonometric identities. Rather sly use of one of the more rare identities today, the ones you want are sum formulas for sine and cosine .
The one in question:
sin (s + t) = sin s cos t + cos s sin t
angle is 20 and nicely enough 10 + 10 equals 20 and 10 is the angle of choice in the other factors.
Hopefully you can see where things go from here.