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Let x=width

x(x+3) = 40
x² +3x = 40
x² +3x -40 = 0
(x+8)(x-5) = 0
x+8=0   ;   x-5=0
  x=-8   ;     x=5

Dimensions are width = 5m and length = 8m .
The other value for x is -8, but you can't use that because negative dimensions don't exist. Length=8m came from x+3
exactly. so, what is the dimension?
5x8 meters
what i mean is where at the two value of x can be the dimension of a rectangle, the 5 right?
Yup :)