Please fast solve .,

Solution
At the condition of just begins to tilt, normal reaction of the cube will shift to its edge as shown.


(Figure shows the situation when the cube is just about to tilt)
At this condition, net torque about point P=0 (for minimum force F)
$$\begin{gathered} ie F\times \left(\frac{3a}{4}\right) = mg\times \left(\frac{a}{2}\right) \\ F =\frac{2mg}{3} \end{gathered}$$