Help with math homework?

Andrew factored the expression -12x^3 -28x^2 -36 as -4x(3x^2-7x+9). But when Melissa applied the distributive law and multiplied out -4x(3x^2-7+9), she got -12x^3+28x^2-36x; thus Andrew's solution does not appear to check. Why is that? Please help Andrew to understand this better. Explain your reasoning and correctly factor the original expression, if possible. If the expression is prime, state so. SHOW ALL WORK