Hello forums, first I want to say thanks for taking the time to help me.

I've taken grade 11 U physics and unfortunately I've been left with a very bad teacher and in a sense I'm left on my own and need help.

My main issue is with the use of equations, mainly in motion ( Newtons 3 laws ). I don't know how to properly re-arrange the equations in order to find the answer I'm looking for since I always seem to get them wrong.

An example : How much force is needed to accelerate a 2.0kg block of wood at 4.5m/s2 along a rough table, against a 10.5N of friction?

The givens I found :

m = 2.0kg
Ff = 10.5 N
a = 4.5m/s2
Fapp = ?

The equation given as a reference was Fnet = Fapp Ff = ma
and after asking the answer to this was Fapp = (m)(a) - Ff

and what I didn't understand is how, how would it turn out like this. Why is (m)(a) not a negative because we brought it on the other side of a equal sign, and where did Fnet go? And why is it being subtracted from Ff, I just don't understand.
Please help and thank you.

Sorry the equation of reference = Fnet = F2pp + Ff = ma