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 – F_friction

When you multiply by mass times acceleration, you are
determining F_total

Thus:

36 Newtons = F_worker – F_friction

We know that the worker exerts a force of 400 N. Thus,

36 Newtons = 400 N – F_friction

What does F_friction have to be to make this work? It has to 
be 364 N, and it has to be opposite the worker’s force.