I'n guessing from your description that you have a force which I'll call F1 with magnitude 700N plus an unknown force F, and that these two forces added together result in a combined force R of 500N in the positivey direction. In other words:
F1 + F = R
To find F simply subtract F1 from both sides:
F = RF1
Please note that F, F1 and R are all vectors, so this requres vector subtraction. Can you finish it from here?
