2(x^{2}-1)=7x+13 solve an sum of two roots
If a(x-b)(x-c)=0,
then ax^2 - a(b+c)x + bc = 0.
The sum of the two roots is the opposite of the coefficient of the linear term divided by the leading coefficient.
Tom's 3rd line is 2x^2 - 7x -15 = 0.
We can immediately say the sum of the roots is 7/2.