Answer
$(3x^3 - b + 2a - 6) + (-4x^3 + b + 6a - 6)$
$= -x^3 + 8a - 12$
Work Step by Step
$(3x^3 - b + 2a - 6) + (-4x^3 + b + 6a - 6)$
$= 3x^3 - 4x^3 - b + b + 2a + 6a - 6 - 6$ (Change signs and group like terms)
$= -x^3 + 8a - 12$ (Combine like terms)