What is a Function? - Definition & Examples

Anna is selling lunch boxes. Each box is sold at $6$ dollars. It costs about $3$ dollars to make the food in each box and the box itself is about $30$ cents. Let $y$ denote Anna's overall profit, and let $x$ denote the number of boxes Anna sold. The following function expresses the relationship between $x$ and $y$:

$$y=(6-3-0.3)x=2.7x$$

Identify the input in this scenario.