I'd put the ones with the same variable(s) together.
In this case, -6x + 3x = -9x
Then, 6+4xy would leave you with that to spare because you cannot add an integer to another integer unless they both have the same variables so that's as simple as it's gonna get which leaves you with... -9x + 6*4xy unless you decided to break it down once more and multiply the integers. That would then leave you with -9x + 24xy .
Well... that's to hard for me sadly when I learned a lot how to do that, i'll give you this though when you going from bigger to smaller you divide, when you going smaller to bigger you times.