Take another look at the sentence:
Agitating the clothes in the sudsy water, the gravy stain on the shirt disappeared
in the wash.
Agitating the clothes in the sudsy water, a participle phrase, comes right before gravy stain, an illogical target. A washing machine, not a stain, does the agitating! Before you continue, review the rules for misplaced and dangling modifiers.
Go back to the sentence to try again.