Question 132226
Note that when the line of the graph is vertical the slope is undefined.

You were given two points:

(1,4) and (r,5)

The slope is undefined.  This means that the value of r must be a value that produces division by zero, which is not allowed in math.

What could be the value of r?

undefined slope = (5 - 4)/(r -1)

The only answer that makes sense is r = 1.  When r is 1, we get a ZERO in the denominator of this fraction, which makes the slope undefined.

How did I get 1 for r?  I set the denominator (r - 1) to zero and solved for r.

r - 1 = 0

r = 1

Got it?  Is this clear?