this is easier than the method maverick did it

just know one concept

slope= (Y1-Y2)/ (x1-x2)

so plug it in

(2,7)

you can make y1=7 so that means x1 = 2

(2,1)

Make y2= 1 , x2= 1

So

(7-1)/(2-2)= 6/0= this is undefined since slope can not be 0

Slope can be zero, but in this case, it is undefined.

If you look at these two points, it is a vertical line; you can see this because the x coordinates are both zero. The slope of all vertical lines is undefined!

If you were to have two points with the same y coordinates (2,3) and (5,3), the graph of this line would look like a horizontal line. The slope of all horizontal lines is zero!

Another way to look at this – if you use the slope formula

(y2-y1)/(x2-x1), if the numerator (top value) is zero, the slope is zero; if the denominator (bottom value) is zero, the slope is undefined.