Which number in the monomial 215x^18y^3z^21 needs to be changed to make it a perfect cube ?

Hello from MrBillDoesMath!

 215

Discussion:

Let's start by rewriting the monomial as

215  x^18 y ^3 z^21

Now..

x^ 18 = (x^6)^3

y^3   = (y    )^3

z^21 =  (z^7)^3

So those terms are already cubes!  The only remaining candidate is the number 215, which is not a cube.

Thank you,

MrB