Question

True or false: Two lines with undefined slopes are parallel to each other and parallel to the y axis

Answer 1

Correct! lines that have undefined slopes are always parallel to the y-axis as the change in x is 0. A gradient is calculated by  the change in y divided by the change in x and anything divided by 0 is referred to as undefined.
 
$\frac{y_{1}- y_{2} }{x_{1}- x_{2} }$

(calculation for a gradient)


Hope that helps sorry if its a bit confusing :)