In order to calculate the missing forces, we need to know that the net force is the sum of all forces in a direction.
For the first situation, the net force is zero, so the sum of forces in each direction is zero.
In the vertical direction, we have:
[tex]\begin{gathered} F_{net}=B-200\\ \\ 0=B-200\\ \\ B=200\text{ N} \end{gathered}[/tex]
In the horizontal direction:
[tex]\begin{gathered} F_{net}=50-A\\ \\ 0=50-A\\ \\ A=50\text{ N} \end{gathered}[/tex]
For the second situation, the net force is 900 N up.
In the vertical direction, we have:
[tex]\begin{gathered} F_{net}=C-200\\ \\ 900=C-200\\ \\ C=1100\text{ N} \end{gathered}[/tex]
In the horizontal direction, we have no forces.
For the third situation, the net force is 60 N to the left.
In the vertical direction, we have:
[tex]\begin{gathered} F_{net}=300-E\\ \\ 0=300-E\\ \\ E=300\text{ N} \end{gathered}[/tex]
In the horizontal direction, we have:
[tex]\begin{gathered} F_{net}=D-80\\ \\ -60=D-80\\ \\ D=20\text{ N} \end{gathered}[/tex]
For the fourth situation, the net force is 30 N to the right.
In the vertical direction, we have:
[tex]\begin{gathered} F_{net}=F-H\\ \\ 0=F-H\\ \\ F=H \end{gathered}[/tex]
In the horizontal direction, we have:
[tex]\begin{gathered} F_{net}=G-20\\ \\ 30=G-20\\ \\ G=50\text{ N} \end{gathered}[/tex]