$x^2 + y^2 =$

$2x^2 + 14x - 15 =$

$a(x + 1) + b(x + 1)^2 =$

$4x^2 - 5x - 4 =$

$xy + 2z + 2z + 4y =$

$x^4 - x^2 - 4x =$

$24 - 3x + 4y^2 - 9x^2 =$

$4 - 9x^2 =$

$a^4 - 81 =$

$ax - 1 + a - x =$