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.

Resolving horisontally,

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

Resolving vertically,

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