This problem uses PGML, with the statement wrapped in BEGIN_PGML...END_PGML. The math delimiters for PGML are different than for regular PG. They are not slash-parens. Here is some documentation:

https://webwork.maa.org/wiki/Mathematical_notation_-_PGML

For inline math, the delimiters are [`...`].

For a comment in PGML, see:

https://webwork.maa.org/wiki/Comments_-_PGML

For more about PGML, see:

https://webwork.maa.org/wiki/Category:PGML_Syntax