The direction of friction force is always the opposite of the direction of the movement.
In this problem, we have an applied force of 20N and this creates a movement. Then, the friction comes in and applies a force of 2.4N.
Because they are in different directions, we need to subtract them:
We don't know the direction because there is no picture (at least I can't see ☺️). But the movement will be to the direction of the 20N.