I love these because you are reversing a formula you already know!
Answer: so you would have to isolate w, and since l,w, and h are all being multiplied by each other, divide by l and h. You now have v over lh = 1/3 times w so divide 1/3 now to get v over 1/3lh = w. Now, plug in v, l, and h to get your answer.