you could solve it by finding the multiples of 21 and 2 that add up to 13, so this gives you (2x-7)(x-3), which you then solve to get x by its self so you get 0=2x-7 and 0=x-3 so you would add to cancel out the 7 and 3 giving you 7=2x and x=3. You found your first root, then for 7=2x you divide 7 by 2 to once again get x by its self giving you x=7/2! Sorry I hope this is easy to understand!
