The slope of diagonal ab is
, and its equation is
.

Given two points (x₁,y₁) and (x₂,y₂) the slope of the line that passes through theses points will be:
m=(y₂-y₁)/(x₂-x₁)

We have two points A(-2,-2) and B(2,-2); therefore the slope will be:
m=(-2+2)/(2+2)=0/4=0

slope-intercept form:
y=mx +b
m=slope
b=y-intercept =-2

y=0x-2
y=-2

The slope of diagonal AB is 0 and its equation is y=-2