Question 764715
Pythagoras.
Width = x
Diagonal = x + 8

Length^2 + Width^2 = Diagonal^2
20^2 + x^2 = (x + 8)^2
400 + x^2 = x^2 + 16x + 64
Collect like terms
16x - 336 = 0
16x = 336
x = 21m
Width = 21m
Hope this helps.
:-)