Anyone here know how to do compound functions, and how to check them?

A university club is planning on asking a company to print custom t-shirts for the 2013-14 academic year.
The quoted price for printing the t-shirts is as follows:
 The set-up fee is $50.
 The first 50 shirts are priced at $12.99 each.
 Any additional shirts are priced at $11.99 each.
Find a formula for the piecewise-defined function that represents the total cost of ordering n shirts from the company. How much would
it cost to order 500 t-shirts?

Question

Anyone here know how to do compound functions, and how to check them?

 A university club is planning on asking a company to print custom t-shirts for the 2013-14 academic year.
The quoted price for printing the t-shirts is as follows:
 The set-up fee is $50.
 The first 50 shirts are priced at $12.99 each.
 Any additional shirts are priced at $11.99 each.
Find a formula for the piecewise-defined function that represents the total cost of ordering n shirts from the company. How much would
it cost to order 500 t-shirts?