Which parabola will have a minimum value vertex?

What numbers are included in the set of integers? what numbers are not included?

Ithink i know it but i want to be sure so can you me out ?

Charlie is at an amusement park that has 18 different rides. each ride costs $1.75. the function c(r) = 1.75r represents the total amount of money that charlie will spend if he goes on r rides, but he never goes on the same one more than once. what domain and range are reasonable for the function?