2x^2+7x=9 can be written as a quadratic: 2x^2+7x-9=0.
This factorises: (2x+9)(x-1)=0, so x=-9/2 or 1. You can see x=1 is a solution by simply substituting x=1: 2+7-9=0. Knowing that (x-1) is a factor it's easy to work out the other: what gives -9 when multiplied by -1? Answer: 9. What gives 2x^2 when multiplied by x? Answer: 2x. So the other factor is (2x+9).