ANSWER: The equation of a line in slope-intercept form looks like the following: y = mx + b
where m is the slope of the line and b is the y-intercept.
Given any 2 points (x1, y1) and (x2, y2) on a line, the slope (m) = (y2-y1)/(x2-x1).
So, Whenever the slope (m) is undefined it means that when you try to calculate the slope given any 2 points on the line you'll end up with a zero in the denominator, which is undefined. That is, with an undefined slope you end up with a vertical line, which has the equation "x=some number".
m = (y2-y1)/(x2-x1) = (y2-y1)/0 = undefined. This implies that x2=x1. So x is the same for any value of y on the line, which is why the equation is "x=some number" and the line is vertical.
So the equation of a line, in slope-intercept form, with an undefined slope (m=undefined) crossing through the point (-3, 5) is: x= -3 , which is a vertical line.
Hope this helps you