In order to find the amount left after both sisters ate the cereal, we have to add the amount of cereal the two sisters ate.
Katrina ate 1 3/4 and her sister ate 7/8.
(1 3/4 + 7/8)
(you put a parentheses around it so that you add the two first before subtracting it from the original amount)
Then, we would subtract it from the original amount: 12 1/2.
-> The equation would be: 12 1/2 - (1 3/4 - 7/8).