$30 is a flat rate charged just for showing up, so that value is constant and will not change.
$15 is the per hour charge, so the total charge will be $30 plus $15 for every hour the tech is there. The $15 is a constant charge per hour, so let x or h represent the number of hours the tech is there, so the total cost is
c = 15h + 30
Every time h increases by 1, the total cost goes up by $15.
