I believe this is the answer since there’s force on the left which is 450 ( a negative 450) and a force on the right 300 ( a positive 300) so overall there’s more force on the left side ( by 150) and you’ll need 150 units to the right to make it balanced or equilibrium.
You start with the bigger number and subtract it because they ´re going opposite directions. It goes to the left because your net force will go in that same direction with the bigger number.
