Actually, you could take 7 out of 6, but let's not worry about that for now. We can add and subtract in any order, addition and subtraction is commutative. so we can add the 6 and the 2, and then subtract the 7

And: 3x-9x-4x+10xy = Is there something I’m missing?

Do you have something missing? Maybe, maybe not (there may be a typo). but this is a workable problem, just add all the x terms together.

3x - 9x - 4x + 10xy = -10x + 10xy or if you prefer, you can factor out the 10x to get: 10x(y - 1)