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