Consider $f(x)=4-e^x$.

A. Find the slope of the graph of $f(x)$ at the point where the graph crosses the $x$-axis.
slope =

B. Find the equation of the tangent line to the curve at this point.
$y =$

C. Find the equation of the line perpendicular to the tangent line at this point. (This is the normal line.)
$y =$