Definitely a WeBWorK bug and not MathJax. Here's what the solution renders as when you set the display mode to tex commands:

\begin{aligned} \frac{4}{y-2} + \frac{7}{2y} & = \frac{4}{y-2}\cdot \frac{y2}{y2} + \frac{7}{2y} \cdot \frac{y-2}{y-2} \\ & = \frac{4y8}{\left(y-2\right)\!\left(y+2\right)} + \frac{7y-14}{\left(y-2\right)\!\left(y+2\right)} \\ & = \frac{11y-6}{\left(y-2\right)\!\left(y+2\right)} \end{aligned} |

The denominator displays (for me) as 2y in the problem. In the solution we can see it start as 2y, then change to y2, and then later to y+2!