Answer
$$ y=-x+5.$$
Work Step by Step
Since the line is parallel to the line given by $ y = 4-x $, then its slope is $-1$; hence the line has the equation
$$ y=-x+c.$$
To find $ c $, we make use of the fact that the line passes through $(2,3)$:
$$3=-2+c\Longrightarrow c=5$$
Hence the equation of the line is
$$ y=-x+5.$$
