Answer :

We know that the formula for cube volume is a³ and that we're looking for a in this specific case, so we can write it down as a simple equation:

a³ = 1000                  / ∛ (both sides)
a = ∛1000
a = 10

Doublecheck:

10³ = 10 * 10 * 10 = 1000, so it's correct ;)