Answer :

Vertical lines, such as this one, have a slope of undefined. 
Hi there, Nichelle! Let's see how well we can explain this. Slope for lines is calculated by finding out how much the line goes up or down the x-axis every time it moves one unit on the y-axis. So to find a slope of a line, we need two points. Let's say (6,2) and (8,3). The first number in each pair of parentheses the x number and the second is the y number. So when we look at it, the y number increases by one unit. In that increase of one unit on the y-axis, the x number increases 2 units. So we would say that the slope is +2. 

All that to get to your actual problem, sorry. So, when we say that a line is x=12 without mentioning the y-axis at all, that means that there isnochange on the x-axis at all for the entire line, the line is perfectly horizontally straight. And such a line has no slope, so we simply say that such lines have 'undefined' slopes. 

If you ever encounter a problem that says x=0, that means that the line is now perfectly straight up and down, or a vertical line. These lines are said to have "no slope".