Answer
a is the input with years as units.
S is the output with dollars as units.
This is not a function.
Work Step by Step
We know that a is the input because it is inside the parenthesis. S is the output because it is the coefficient of the parenthesis. The units of years for a and dollars for S are both given in the problem. This is not a function because for any one age, different people could have different salaries.
