Please help me! I need help on solving this question

From the question, we were told that:
The restaurant offers a catering service which costs $16.50 per person with a service charge of $86.00
For parties of 75 or more people, a group discount is applied and the cost per person is then $13 and service charge also dropped to $43.
We are asked to write a piecewise-defined linear function which calculate the total cost T of the catering service which serves n people.
The piecewise-defined linear function of the above question is:
T(n) = 16.5n + 86; 0 ≤ n ≤ 75
= 13n + 43; n ≥ 75