We know that C is dependent on how many hours of service was done. We also know that C is linearly dependent on how many hours of service was done because it's a constant $50 per hour.
So, the equation would be a linear equation in the form C = 50h + L where h is the number of hours and L is the y-intercept.
Since there is an initial charge of $25, when h = 0, C = $25 -> L = $25.