The legth of a rectangle is three times its width. if the width is diminished by 1 m and the legth increased by 3 m, the area will be 72 sq m. find the dimensions of the original rectangle?

Let width of rectangle be "x"
L= 3x W=x
if width is diminished by 1 that means new width = (x-1)
similarly the new length would be (3x+3)
hence (x-1)*(3x+3)=72
(3x^2)-3=72
3x^2=75
x^2=25
x= +/- 5
reject -5 as you cant have negative length
hence x=5
so original dimensions of rectangle is 
L= 15m W=5m