Answer:
Mary's answer was correct because it can be factored into a product of binomial times a trinomial. The reason is that they are similar in structure. In addition to the “sum” case, the middle sign of the trinomial factor will always be opposite the middle sign of the given problem. Therefore, it is negative.
Step-by-step explanation:
[tex]\mathrm{125x^3+216}\\\\\mathrm{=\left(5x\right)^3+6^3}\\\\\mathrm{=\left(5x+6\right)\left(5^2x^2-6\cdot \:5x+6^2\right)}\\\\\mathrm{=\left(5x+6\right)\left(25x^2-30x+36\right)}[/tex]