Chapter 2 - Force Vectors - Section 2.3 - Vector Addition of Forces - Problems - Page 32: 26

$\theta = 54.3°$ $F_{A} = 686 N$

By projection of both forces on the x axis, we obtain $F_{x} = F_{A} \times \sin(\theta) + 800 \times \cos(30°) = 1250$ Now, applying the same logic on the y axis $F_{y} = F_{A} \times \cos(\theta) - 800 \times \sin(30°) = 0$ By solving the two equation system we obtain that $\theta = 54.3°$ $F_{A} = 686 N$