The following problem comes from "Vector Mechanics for Engineers: Statics" from F.Beer
The solution goes on with law of cosines to find $P$ for item (a) and law of sines to find $\beta$ and then find $\alpha$ for item (b). What I don't understand is why the angle between the $600$N and $1200$N forces is $85°$, I thought it should be $180°-35°-50°=95°$. So, I want to know why the angle between the tension forces is their sum and not what I've thought in the first place.
In the force diagram, the arrows are placed so that they follow each other. This is because the total vector sum is zero since the system is in equilibrium.