First on the left:
Net force = zero
That tells you that horizontal forces add up to zero,
AND vertical forces add up to zero.
Horizontal: 50 right + A left = zero
A = 50 left
Vertical: 200 down + B up = zero
B = 200 up
Second from the left:
Net force = 900 up
This tells you that vertical forces add up to 900 up.
Vertical: 200 down + C up = 900 up
Add 200-up to each side: 200-down + 200-up + C-up = 1100 up
C-up = 1100 up
Third from the left:
Net force = 60 left
This tells you that the horizontal forces add up to 60 left,
and the vertical forces add up to zero.
Horizontal: 80 left + D right = 60 left
(D-right = negative D-left)
80-left - D-left = 60-left
Subtract 80-left from each side: - D-left = -20 left
D-left = 20 left
Vertical: 300 up + E down = zero
Subtract 300-up from each side: E down = -300 up
E down = 300 down
Right-side picture:
Net force = 30 right
This tells you that the horizontal forces add up to 30-right,
and the vertical forces add up to zero.
Horizontal: 20-left + G-right = 30 right
Add 20-right to each side: 20-left + 20-right + G-right = 30 right
G-right = 30-right
Vertical: F-up + H-down = zero
F = H
