##(4x-16)cms##

Let the length be ##color(red)(x##

Then

The width will be written as ##color(red)(x-8##

(Width of rectangle is ##8cm## less than length )

Perimeter of rectangle=##color(blue)(2(l+b)units##

SoSubstitute ##color(red)(x## and ##color(red)(x-8##

##rarr2(color(red)((x)+(x-8)))##

Remove brackets

##rarr2(color(red)(x+x-8))##

##rarr2(color(red)(2x-8))##

Use distributive property ##color(brown)(a(b+c)=ab+ac##

##rArrcolor(green)(=4x-16cms##