Question 51603
The slope m of a line is defined as {{{m=(y2-y1)/(x2-x1)}}}, where (x1,y1) and (x2,y2) are two points on the line.  Now we substitute the points (4,0) and (4,5) into the equation -- note it doesn't matter which ordered pair is 1 or 2, as long as you're consistent.  The equation becomes {{{m=(5-0)/(4-4)}}}, which simplifies to {{{m=5/0}}}.  Because division by zero is undefined, our slope is undefined.  By definition, an undefined slope means a vertical line; a slope of 0 means a horizontal line.