Hello! try simplifying those two numbers so that they have the same denominators. You can see that you can convert 11/2 of flour so that you can add it with 3/4 of sugar and get the total amount together.
To convert multiply the top and bottom by 2

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Now add across the top numbers

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And finally simplify the fraction to get your final answer. You can pull out 24 out of 25 as the common factor for 4 and 6

and the remainder will be in the numerator.