The members of a truss are pin connected

The members of a truss are pin connected at joint O. Determine the magnitude of $F_1$ and its angle ϴ for equilibrium. Set $F_2$ = 6 kN.

Image from: Hibbeler, R. C., S. C. Fan, Kai Beng. Yap, and Peter Schiavone. Statics: Mechanics for Engineers. Singapore: Pearson, 2013.