$${\begin{align*} a _{\text{mk},x} &= \frac{F_{1x} + F_{2x} + F_{3x}}{M} \\&= \mathrm{\frac{-0,6\, N + (12\,N) \cos 45^\circ + 14\, N}{16\, kg}} = 1,03 \, \mathrm{m/s^2}\end{align*}}$$