Then give this piece of paper, together with the original unaltered homework/test solutions, to the instructor within one week of when the graded item was first available. All regrading requests must be done in writing -- you may not argue your grade in person.
Your claim will then be examined. If it is found to be valid, then an appropriate adjustment to your mark will be made. The regrading decision of the instructor is final.
WARNING: If your regrading request is found to be spurious or unconvincing, then the instructor may also choose to re-examine the grading of your entire homework or test, with a critical eye, and may decide to deduct various points. So, your mark may end up going down rather than up. Thus, you should only request a regrade in cases of genuine, clear grading error (e.g. incorrectly adding up the marks, or marking a solution as wrong when it is actually correct). You should not submit regrade requests which are merely a matter of "judgement" about how many points a partial solution is worth. You have been warned!