Multiply:

(3x-4)²
It would be (3x-4)(3x-4).

Would the answer be 9x²+16?

No, you are missing the middle term

You need 3x(-4) + 3x(-4) = -24x

(3x+4)(3x-4) = 9x^2 -16 This is the case where there wouldn't be a middle term.