The equation of a line starting from two points is:

From the first point you get: x1 = -1, y1 = -2
From the second point you get: x2 = 3, y2 = 10
Replace x1, y1, x2, y2 in the equation of the line and you get:



From this you get the equation of your line:
The width is 18.
step by step:
1. 2 lengths are 20 which is 20+20 that's 40.
2. 76-40= 36
3. Two widths are 36 cm.
4. so do 36÷2 and that equals 18
4 x -5 = -20 then 12 + -20 = -8