alright so, since it says that he eats at least $10 dollars no matter what we can basically leave that alone. next week have $3.50 for every new customer (x) so we could write this as $3.50x. and lastly we have Y the amount he will earn every week.
so the equation would be $10+$3.50x=Y.
if we had more information we might be able to solve for Y but since it doesn't tell us how many he sold we would leave it like that.