3x + (-2+4x)

what is the answer?

I am assuming you are meant to factor this.

Distribute 3x to the parentheses.

3x*-2 = - 6x

3x*4x = 12x^2

Put it all together, largest sums first.

12x^2 - 6x

Not sure if they want you to simplify it further by dividing the common factor, but most likely not since you just expanded it. You'd be going backwards if so, and you should challenge your instructor on their logic.