Undefined slope means it is a vertical line.
If it is a vertical line the only number that matters from the point given is the x-value because the line would pass every y number on that x-value.
Equation of a line : y=mx+b
m=slope and b= y-intercept
There is not y-intercept because it is vertical and only passes through x, therefore the equation is x=5
4 1/2 = 9/2
1 2/5 = 7/5
9/2 dived by 7/5 = 9/2 x 5/7
= 45/ 14 = 3 3/14