When you talk about square roots, you are really talking about raising something to the 1/2 power. In other words, the exponent will be 1/2. So first we need to convert the square roots for both variables (y and z) into numerical exponents that we can then simplify.
`sqrt(y^12)sqrt(z^18) = y^(12/2)z^(18/2)`
We can then simplify the fractions in the exponents to whole integers.
`y^(12/2)z^(18/2) = y^(6)z^(9)`
Since y and z are two distinct variables, this is the simplest form of the expression.