M is directly proportional to L3. How many times larger is M when L is multiplied by 2?
Is this what you mean?
\(M=kL^3\\ \mbox{Now look at what happens to the right hand side if L is doubled}\\ k(2L)^3=8kL^3\\\)
So when k is doubled, L becomes 8 times greater. (That is 2^3 greater)
Oh, you should have written L3 as L^3 :)
