Forces and equilibrium — practice question

At point O, which is the origin of the x-axis and y-axis, three horizontal forces of magnitudes $F\,\text{N}$, $63\,\text{N}$ and $25\,\text{N}$ are acting. These forces are in equilibrium. The force with magnitude $F\,\text{N}$ is inclined at an angle $\theta$ anticlockwise to the positive $x$-axis. The force of magnitude $63\,\text{N}$ is directed along the negative $y$-axis. The force of magnitude $25\,\text{N}$ is directed at $\tan^{-1}0.75$ clockwise from the negative $x$-axis (see diagram).