t is the input (the year) with years as the units. T is the output (amount of taxes) with dollars as the units. This is a function.
Work Step by Step
We know that t is the input because it is inside the parentheses. T is the output because it is the "coefficient" of the parentheses. The units of years and dollars are both given in the problem. This is a function because there can only be one quantity of taxes that you've paid in any given year.