Answer:
there would be <u>8</u><u> </u><u>sides</u> if there are 20 diagonals
Area of Rectangle = length times width
lets say length = 2x-4
width = x+5
Note: you can switch these around, width could be 2x-4 and length could be x+5, it doesnt matter.
With that being said:
length * width = area
(2x-4)(x+5)=area
FOIL (First Outer Inner Last)

Simplify: