(x + 3)^2 - (x - 1)^2 - 2x^2

(x + 3)^2 - (x - 1)^2 - 2x^2 =

(x^2 + 6x + 9) - (x^2 - 2x + 1) - 2x^2 =

x^2 + 6x + 9 - x^2 + 2x - 1 - 2x^2 =

6x + 9 + 2x - 1 - 2x^2 =

8x + 8 - 2x^2.