If $\displaystyle sin (\alpha) = -\frac {5}{13} $ and $\displaystyle tan ( \alpha) > 0 $ , find the exact value of $\displaystyle sin (\alpha - \frac{\pi}{3}) $.

My first time using latex! Hopefully it makes my problem easier to read.

EDIT:

so the formula is sin(A-B)=(sinA)(cosB)-(cosA)(sinB)

I guess I should mention I know how to get every value except $\displaystyle cos(\alpha)$

and I don't understand why they are telling me $\displaystyle tan ( \alpha) > 0 $