If a roll is 6′ wide with a total area of 84 square yards, what is the length?
I think this is really a math problem and not DIY. But if this is referring to buying carpet or other roll products for a house then it could by DIY:


Area = Width x Length which is the same as Length = Area / Width
1 yard = 3 feet


Total area = 84 square yards = 84 x (3 feet/yard) x (3 feet/yard) = 756 square feet

Width = (756 square feet) / (6 feet) = 126 feet

42 is the Answer to the Ultimate Question of Life, the Universe, and Everything but it is not the correct answer to this math problem.

I suspect that somewhere you multiplied or divided by 3 (== feet per yard) instead of 9 (== square feet per square yard), as 126 / 3 = 42.