Well, to find the slope of a line, mx + b = y is the formula, and m is slope. 
-71 - 2y = 56
We add the 71 on both sides. 
-2y = 126
Divide by -2 on both sides.
y = -63
There is no distinguishable slope in this because y equals an integer, not an expression that includes x or m, so the slope is zero. 
Hope this helps! (: