Let width,w =x
Now it says that length is 6 less than twice width
twice of width means 2x
6 less than twice width means 6 less than 2x,thus we have 2x-6
so length,l =2x-6
Given, perimeter =90
The formula for perimeter,P =2l+2w
So we plug in value of w and l into it and P=90
2l+2w =P
2(2x-6) + 2x =90
4x-12 +2x =90
6x -12 =90
6x= 12+90
6x=102
x=(102/6)
x=17
so width = x =17
length =2x-6 =2(17)-6 =34 -6 =28