Typically a good place to start with problems like these is just to write down every piece of information you're given and then see what else you can...
Oh, whoops! Yes, I made a small typo. Sorry about that. I'm glad you figured out what I meant though, and it didn't lead you too far astray. I'll go...
Well, you ended up with the right answer, so that's good. But your method is absolutely incorrect, and it's very unclear as to what you actually did....
Generally speaking, maximization/minimization problems are in the domain of calculus. Have you learned any calculus techniques, such as derivatives,...