Two forces can add or subtract to the net force when the forces act on each other. Forces in the same direction working togeather equal a net force.
\( F_n \;=\; F_1 + F_2 \) (Net Positive Force)
\( F_1 \;=\; F_n - F_2 \)
\( F_2 \;=\; F_n - F_1 \)
\( F_n \;=\; F_1 - F_2 \) (Net Negative Force)
\( F_1 \;=\; F_n + F_2 \)
\( F_2 \;=\; F_1 - F_n \)