Answer:
B
Step-by-step explanation:
It's B, because it's the only way to find n on both sides
12x^4 - 42x^3 - 90x^2 =
6x^2(2x^2 - 7x - 15) =
6x^2(2x+3)(x - 5) <==
We have such equation
6x+3=4x+9 /-4x subtract 4x both sides
6x-4x+3=4x-4x+9
2x+3=9 /-3 subtract 3 both sides
2x=6 /:2 divide both sides by 2
x=3 - its result
