A grocery sells green apples and red apples. On Monday, the store put 500 of each kind of apple on display. That day, the store sold 42 red apples and 57 green apples. On Tuesday, the store sold 87 red apples and 75 green apples. On Wednesday, the store sold 29 red apples and 38 green apples. a. Write an addition expression using negative integers and find the number of red apples the store sold.