$${\begin{align*} a _{\text{mk},y} &= \frac{F_{1y} + F_{2y} + F_{3y}}{M} \\&= \mathrm{\frac{0 + (12\,N) \sin 45^\circ + 0} {16\, kg}}= 0,530 \, \mathrm{m/s^2}\end{align*}}$$