Polynomials are linear combinations of positive integer powers. For example,

is a polynomial, but

is not.
(In case you're unfamiliar with the term, a linear combination of two quantities is the sum of scaled versions of those quantities, e.g. a general linear combination of
and
is
, where
are any numbers.)
is not a polynomial; the offending term is
, or
.
It adds up to 180 degrees
$2600 per month earning
$520 per month on food
Her monthly income spent on foo is about $520.
12-x(5-4k)=y
12-y=x(5-4k)
12-y/(5-4k)=x