The only choice that multiplies out to give the original expression is ...
... b. 3x(x + 4)(x − 2)
_____
After you factor out 3x, you have 3x(x² +2x -8). At this point you can eliminate answers C and D because the product of the constants in the binomial factors is not -8. You can eliminate answer A because the product of binomial terms gives -2x as the linear term in the quadratic factor, not +2x.
