When you do a few of them, you can do it easily in one step but in the meantime, the way you suggested is good:
3-2x-x^2=-(x^2+2x-3)=-(x+3)(x-1)=(x+3)(1-x)
An easy way to confirm that your answer was wrong was that you had (..-x)(..-x) which would give x^2 when multiplied out not -x^2.