equations

$\mbox{I'm going to assume you mean }\\3x^2 + 2(x^2-20)=16 - 2x^2$

$3x^2+2x^2-40 = 16 - 2x^2 \\ \\ 7x^2=56 \\ \\ x^2 = 8 \\ \\ x = \pm \sqrt{8} \\ \\ x = \pm 2\sqrt{2}$