Part 3: If you sub b into the original equation, it must turn out to be 0 (since b is a root). So b^2-3b-13=0. Now multiply each side by -2, you get -2b^2+6b+26=0. Now subtract 26 from both sides and rearrange the LHS to get 6b-2b^2=-26. So you can see the LHS is exactly what the question wants...