Hi there!
Thanks for asking for help!
I believe Part A of your answer is the expression: n+2n.
Because that is the answer for Part A, Part B is the answer: "The expression for the bookcases is n+2n because the problem says 'the larger bookcase holds twice as much as the smaller one,' and 'let n represent the amount of books the smaller bookcase holds'.
Since you need to find an expression for the amount both bookcases needs to hold you would add n (capacity of the smaller bookcase) and 2n (capacity of the larger bookcase) together to have an expression that represents the amount both hold.
Part C: 90=3n
I know this is the correct equation based on the expression written above since n+2n=3n, and the problem says "the amount both bookcases hold together is 90 books"
btw, bookcase 1 holds 30 books and the large bookcase holds 60.
Hope this helps you out! :)
