Trigo.

Well, since you didn't give us angles we can't really help you lol..because;

Sin is a/c and cos is b/c, and finally tan is a/b but without the 3 angles of the triangle(a,b,c) it's hard to help buddy

>>7794613
OP is a faggot as usual: >>7794594

>>7794613
This belongs in /sqt/ but I'll bite. Since the tangent is negative and cos(b)>0, sin(b)<0. let cos b = c. sin(b) = -sqrt(1-c^2), the negative value.
Solve for c in this: tan(b) = -sqrt(1-c^2)/c = -3/4
then evaluate sin (2b) using the double angle identity.