Alright, let's use regular numbers to explain.
3(5) + 4(5) = 15 + 20 = 35, but, it also equals (3 + 4)5 = 7(5) = 35. It would be similar with 3(5)2 + 4(52).
Sooooo, -3x2 - 6x2 = -9x2
If the exponents were different, they would not be like terms. -3x2 - 6x cannot be combined, for example, but -3x100 - 6x100 = -9x100.
Hope this helps!
