The important thing to remember is that (mass) x (acceleration)
is equal to the TOTAL force.
What is going on in the example? The worker is pushing, and
friction is resisting. Thus, there are two forces. They
work opposite each other, so the TOTAL force is
F total = F worker - Ffriction
When you multiply by mass times acceleration, you are
36 Newtons = Fworker - Ffriction
We know that the worker exerts a force of 400 N. Thus,
36 Newtons = 400 N - Ffriction
What does Ffriction have to be to make this work? It has to
be 364 N, and it has to be opposite the worker's force.