Let n = the number of apples the store has.
"The grocery store has 215 green apples and 138 red apples "
The total number of apples is: 215 + 138
"On Monday they sold 172 apples"
The total went down 172.
n = 215 + 138 - 172
n = 353 - 172
n = 181
Answer: The grocery store has 181 apples left.