Question

A line with an underfined slope contains the point (8,3). The point (?, -4) is also on the line. What is the missing x-coordinate?

Answer 1

Do you mean undefined?
Remark
If a line has an undefined slope, it is of the form x = a

So in your case, the equation of the line is x = 8. The y value only completes the point. 

So - 4 completes the point of (8,-4) The answer is 8