The amount of rhubarb in the original recipe is 3 1/2 cups. Using what you know of whole numbers and what you know of fractions, explain, how you could triple that mixed number.

The easiest way, I think, is to convert the mixed number into an improper fraction, then multiply by 3.

3 1/2 = 7/2

7/2 · 3 = 21/2

now just change the improper fraction back to a mixed number by dividing and putting the remainder into fraction form

21/2 = 10 1/2

You could also multiply the whole number by 3 and the fraction by 3, ending up with 9 3/2, but then have to convert the improper fraction into a mixed number

3/2 = 1 1/2

then add the numbers together

9 + 1 1/2 = 10 1/2

either way works, whatever is easiest for you.