I'm having trouble with this question. I was thinking of using Lami's theorem, but its not helping too much.

A body of mass 5 kg is in equilibrium when it is acted upon by three concurrent coplanar forces P, Q and R as shown in the diagram below. P = 10 newtons and Q = 20 newtons. The angle between P and Q is 170 degrees and the angle between P and R is $\displaystyle \theta$ degrees

Find $\displaystyle \theta$.