The ingredients given are for 4 dozen cookies; however, Christian only want to make one dozen, meaning he requires only 1/4 the ingredients.
1/4 of 2 1/4 cups of while sugar is found the following way.
First, we write 2 1/4 as an improper fraction
1/4 of this is
Hence, Christian requires 9/16 cups of white sugar.
Similarly, the 1/4 of 3/4 cups of brown sugar is
Hence, Chrisitan requires 3/16 cups of brown sugar.
Therefore, the total cups of sugar required is the sum of
3/4 cups of sugar.