Take another look at the 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.

Go back to the sentence to try again.

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