let the roots be x and y. we know that x+y=-b/a and xy=c/a. the sum of the squares of the roots is x2 + y2 and we want to rearrange this to be a comination of (x+y) and xy. (x+y)2 = x2 + y2 + 2xy. therefore, we can write x2 + y2 as (x+y)2 -2xy. subtitute in the other answers from the sum of the roots and the product of the roots gives you (-b/a)2 -2(c/a). in the quadratic 18x2 +21x-400, a=18,b=21,c=400. substitute those values in and you get (-21/18)2 -2(-400/18). hope that helps.
