When you and your friend push a table in opposite direction, it becomes difficult to move. This is because the net force becomes the difference of the two forces.
Whenever forces act on an object in opposite directions—like the book on the table—the net force is equal to the difference between the two forces. In other words, one force is subtracted from the other to calculate the net force. If the opposing forces are equal, or balanced, the net force is zero, as it is for the book.
Hence, the net force becomes the differences of the two forces.