The slope of the line is essentially it's rate of change - the ratio of the change in its y-value to the change in its x-value. We can find both of these changes by subtracting one value from another. The two y-values we're given are 11/7 and 4/7, so their change is 11/7 - 4/7 = 7/7, or 1. Our two x-values are 2/3 and 2/3, so their change is 2/3 - 2/3 = 0.
The ratio between these two changes is 1/0, but since division by 0 isn't allowed, the ratio - the slope - would simply be undefined.
