You can put this solution on YOUR website!
I don't have that book available to me, but there are two things they could be asking you to do:
Group: 5x(x-7) + 3(x-7)=(5x+3)(x-7)
or expand: 5x(x-7) + 3(x-7)=5x^2-35x+3x-21=5x^2-32x-21.
I am inclined to think that they wanted you to group. If you need anymore help, send me an E-mail via my me page, or to email@example.com. I am happy to help and give more in-depth answers.