a^2b^2x^2- (4b^4 - 3a^4)x -12a^2b^2 = 0
⇒ a^2 b^2 x^2- 4b^4 x + 3a^4 x -12 a^2 b^2 = 0
taking xb^2 (first term below)and 3a^2(second term below) outside,
⇒ b^2x(a^2 x - 4b^2) + 3a^2(a^2x - 4b^2) = 0
⇒ (a^2 x - 4b^2)(b^2 x + 3a^2) = 0
⇒ (a^2 x - 4b^2) = 0 or (b^2 x + 3a^2) = 0
⇒ a^2 x = 4b^2 or b^2 x = - 3a^2
⇒ x = 4(b^2)/(a^2) or x = - 3*(a^2)/(b^2)
hope it helps