"Reading" the Diagram
In this problem, you want to balance the two given force by adding two forces to the object. The given forces are angled forces. Like all angled forces, they have x- and a y-components. Since the forces are drawn on a background grid of squares that represent 10 N along each edge, it is easy to determine the x- and the y-components. For instance, an angled force vector that stretches horizontally for 7 squares has an x-component of 70 N. If the same angled vector stretches vertically across 4 squares, then the y-component is 40 N. Begin this problem by determining the x- and the y-component of the given force.
"Vector Summing" the Forces
The next step is to add the components of the two given forces. Components are vectors too so when you add components you must consider their direction. Add the x-component of Vector A to the x-component of Vector B to get the toal amount of x-Force provided by Vectors A and B. Repeat the process for the y-components; add the y-component of A to the y-component of B to determine the total amount of y-Force provided by Vectors A and B.
Balance the Forces
If the two given forces have a combined (sum) x-component of 70 N to the west, then you need to add an equal strength force in the opposite direction. That is the only way that you can balance the given forces. So add a force of 70 N to the east. By the same reasoning, you will have to add a vertical force to balance the combined (sum) y-component of Force A and B. So if the combined y-component is 40 N to the north, then add a 40 N force directed southward.