I'm unsure which part is unclear to you, but Ill break it down as much as possible. You have two forces of size/magnitude of 3N and 4N. They are said to be acting on the body perpendicularly. Perpendicular means that the force is acting on the object at a 90 degree angle with respect to the surface the object is on. Say we have a block on a table. The two forces are acting down on the block pushing it towards the table. Since both forces point in the same direction, they can be added together. There is a resultant force of 7N in the negative y-direction
