X = number of minutes Maria spends talking on the phone each month y = Maria's monthly cost
If she talks for x minutes, at $0.01 per minute, then she is charged 0.01*x dollars for those x minutes. This is on top of the $39,99 as the flat fee (which is charged no matter how many minutes are used; even if Maria doesn't use her phone at all)
So that means we add 0.01*x to 39.99 to get 0.01*x+39.99 This expression is equal to y as it is the total cost for that month
Therefore y = 0.01*x+39.99 is the equation If you want things in function notation, then you'd replace y with f(x) to get f(x) = 0.01*x+39.99
