if the area of a rectangle is width time length, then we have all of the info we need.
the width is 6 less than 3 times the length. let the length be x and put your words into math! 6 less is -6. 3 times a number is 3x, so width=3x-6.
so we have area=l×w which becomes x×(3x-6)=105
solve

factor and solve!
you will get one positive and one negative. pick the positive since lengths cannot be negative