You know how to add and subtract, so why not do what you already know how to do? What is 3 - 6? If you get -3, then you've already solved half this problem.
Next, what happens if you combine -4.2x and 2.25x? Line them up and combine (add or subtract) them vertically. Is this addition or subtraction?
Write out your two results. You will obtain a negative term in x and a negative constant term.
