Alarge rectangular movie screen in an imax theater has an area of 6,461 square feet. find the dimensions of the screen if it is 20 feet longer than it is wide.
length

Width= 71ft
Length=91ft
In this case you are given the rectangular area ( 6,461 square feet) and the length to width relation (length= width +20 ft). Since area is length multiplied by width, then the equation would be:

Area= length x width
Area= (width+20ft) x width
width^2 + 20width - 6461 =0
(width+91 ) (width-71)

Then width should be either 71 ftor (-91)ft. Since width is not possible minus, it definitely 71ft. Then length would be
Length= width+20= 71 +20= 91ft