Here is the original item:
Wendy pushed her glasses up her nose,
(A) and blew
loose strands of hair from her eyes,
(B) hoping
that clearer vision would help her make sense of the difficult algebra
problem (C)
that she was trying to solve.
You wanted to do this:
Wendy pushed her glasses up her nose, and
blew loose strands of hair from her eyes,
hoping that clearer vision would help her make sense of the difficult algebra
problem,
that she was trying to solve.
A comma after problem is a problem. That she was trying to solve is a subordinate clause. When a clause of this type concludes a sentence, you do not use a comma to attach it.
You might want to review the rules.