Hello! I'm in a play in 3 weeks and I need to lose some weight to fit into my costume. I'm a 5'10 17yo female and I weigh 174 lbs. I have 3 weeks to drop 9 or 10 pounds. How can I do this healthily?

Hello there and congratulations on acting in a play!  How exciting for you!

Losing weight is a game of patience.  Our bodies gets used to the weight we currently are at and fights to lose weight, which is why it usually takes time to lose weight.  A healthy weight loss can be 0.5 lb to 2 lbs/week.  This page from an older question on losing weight may give you some ideas on how to lose weight eating a balanced diet.

Good luck!