I am told that$\displaystyle x= -7/25$ and $\displaystyle y = 15/8$, where x is in the second quandrant and y is in the third quandrant.

I need to find the exact value of:

$\displaystyle sin(x-y)$

I know I need to use the difference formula some how and I realise that sin is negative in the 3rd and 4th quadrant; however, I didn't know how to apply the negative sign for $\displaystyle y$. Do I apply it within the sin function or does it make the entire sin function negative? The confusion was because there are two values in different quadrants.

Also, how would I go about converting x and y into values that I can derive exact values from? Do I need to rearrange them before I apply the sum/difference formula or after?

Thanks.