we are given
You can buy a 6 hour phone card for $5
so, total hours = 6 hours
now, we can change it into min
6 hours = 6*60 min

Let's assume
x is the number of actual minutes talked,
y is the called number of minutes left on the card
and we are given
each minute you talk actually costs you 1.5 minutes of time
so, we get equation as

Graph: