The Starfish Diner serves 6 customers. Each customer buys a Crispy Star for 3 shells and a glass of milk for 1 shell.

The Starfish Diner serves 6 customers. Each customer buys a Crispy Star for 3 shells and a glass of milk for 1 shell.

One customer has a coupon, so their glass of milk is free.

Which equation can we use to find  the total number of shells, the diner collects?