Newton's first and second law lecture

To determine the direction and magnitude of net force and thus acceleration (because force is mass times acceleration; F=ma), you have to perform vector addition as forces are vectors. To be able to do this, you'll need two pieces of information; the vectors' magnitudes and direction.

When drawing vectors, vectors of a greater magnitude are drawn longer. Thus, by looking at the lengths of the forces in the free-body diagram, you can determine which force is larger and which is smaller. To take the provided image as an example, Force Fa is larger than force Ff as its arrow (vector) is longer - it has greater magnitude.

Their direction, on the other hand, can be determined by just looking at what direction they point in. Vectors pointing in opposite directions subtract, while ones pointing in the same direction add. If they perpendicular, they form the sides of a right triangle and the resultant vector is the hypotenuse. If they're at some other angle, then things get a bit more complicated, although that's beyond the scope of this answer (feel free to leave a comment if you're curious though (or just look around khan academy))

So, to return to the question at hand;

By looking at the free-body diagram we can see Ff is shorter than Fa. Despite pointing in opposite directions, the force does not cancel out completely as they are of different magnitude. Thus, the force in the x direction is non-zero. So is acceleration, as F=ma.

Fn and Fg are of the same length, and point in different directions. They cancel completely and thus the net force and acceleration are both zero.

Hope this helps!