I'll explain why, and you can put it in a two column proof as you wish due to that I don't know how to do them :P
Expanding it, we get x^2+3x-15=3x+10. After that, we can subtract 3x from both sides as well as subtract 10, getting x^2-25=0, and x^2=25. Therefore,
x= +-sqrt(25)=+-5