Answer :

the formula would be y₂-y₁ ÷ x₂-x₁
-1 :x₁
4 : y₁
                                            4-4/5-(-1)     = 0 
5 : x₂
4 : y₂

0 is the slope
your line is horizontal (same y values) in horizontal lines, the slope is always 0, so, your slope is 0