Do you know what the sine and cosine rules are? Because if you do it's simply just a matter of plugging the values into the equations.
The sine rule is
So if you take to be the side QS then would be , would be 15, and would be (as )
After solving this you'll have the side length you need to apply the relevant cosine rule to obtain the length of QP.
Let us know if you have trouble with this.