From Mechanics

The sum of the forces is zero as the body has constant velocity. This means the sum of the forces in all directions is zero.

Horisontally,

\displaystyle \begin{align} & \to :\quad 12\ \text{N}-P=0 \\ & \\ & P=12\ \text{N}\ \\ \end{align}

Vertically,

\displaystyle \begin{align} & \uparrow :\quad 45\ \text{N}-Q=0 \\ & \\ & Q=45\ \text{N} \\ \end{align}