Like terms are when the variable you have are the same. In this case it is x. Therefore, when you have like terms, you should add, or subtract your like terms.
(note: When you move one thing to the other side of the equation, make sure to do the opposite of the sign. i.e. if it is +4 when you move to the other side it will be -4).
15x+12=45+4x
15x+12-12=45+4x-12
15x=45+4x-12
15x-4x=45+4x-4x-12
15x-4x=45-12
11x = 33
this is the same as:
11*x = 33
the opposite of multiply is divide so when we move the 11 it has to be divide
11/11*x=33/11
x=3