Final answer:
The expression 21a + 9b can help Braedyn find the total number of items on the shelves by multiplying the number of short and tall shelves with the number of items on each shelf.
Step-by-step explanation:
The expression 21a + 9b can help Braedyn find the total number of items on the shelves by multiplying the number of short shelves (21) by the number of items on each shelf (a), and multiplying the number of tall shelves (9) by the number of items on each shelf (b). Then, sum up the results of both multiplications.
For example, if each short shelf has 5 items and each tall shelf has 3 items, the expression would be: 21(5) + 9(3) = 105 + 27 = 132. So, there would be a total of 132 items on the shelves.