What people struggle to understand, is that letters just represent numbers, same for pretty much any symbols, later they will probably use greek symbols and stuff to throw you off but its all the same stuff.
At that level, any letter represents any real, constant number.
So instead of writing 6 x 2 = 12
you can write A x B = C
when A= 6, B= 2 and C= 12
so don't get confussed by it all. If you have an unknown variable isolate it and then work it out. i.e.
2 + 2A = 18 (first of all move the 2 across - remember you can put anything from 1 side of an equals sign to the other if you switch the sign)
2A = 16 (then you can divide both sides by 2 so that)
A=8
SIMPLES