Like terms means that the numbers have the same variable after them or they don't have a variable at all. It is also important to remember that any letter that doesn't have a number in front of it technically has a 1 in front of it.
For example, x + 3x = 1x +3x. Then I add the numbers in the front and I get 4x.
If it had been 4x + 7x + 3, my answer would have been 11x +3. That's because there were a total of 11 x, but the 3 ones weren't x's so I just leave them alone.
