\begin{align} ΣM_S=0 =&\cssId{Step1}{M_A - M_{mg} - M_{mpg}}\\[3px] &\cssId{Step2}{F_A * x_{min1} - mg * x_{min2} - m_pg * x_{min3} =0}\\[3px] &\cssId{Step3}{F_A = \frac{mg * x_{min2} + m_pg * x_{min3}}{x_min1}}\\[3px] \end{align} (_S = suport) (m = person's mass) (m_p = planck's mass)

1. In the first step we make the summation of momentums from point S (suport)
2. In the second step we express every momentum as the product of the force (F_A, mg, etc.) and the minimum distance, of these forces, from point S (x_min1,x_{min2, etc.)}
3. In the third step we isolate F_A to one side of the equation and everything else to the other side so that you can solve the equation