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".