Answer:
undefined 
Explanation:
the equation for the slope of the line is (y2-y1)/(x2-x1) 
here are the given points: (-6,-8) and (-6,12) 
label the points: 
x1=-6
y1=-8
x2=-6
y2=12 
now substitute into the equation (m is slope) 
m=(12--8)/(-6--6)
m=(12+8)/(-6+6) 
m=20/0 
m=undefined
the slope is undefined because you cannot divide by 0
Hope this helps!