## Precalculus: Concepts Through Functions, A Unit Circle Approach to Trigonometry (3rd Edition)

$N(x) = I(x) - T(x)$
After tax income (N($x$)) is given by the total income (I($x$)) minus the total tax paid (T($x$)), thus N($x$) can be expressed by $N(x) = I(x) - T(x)$