To combine like terms in the expression \(6y^3 + 5z^2 - 5z^2\), we can see that \(5z^2\) and \(-5z^2\) are like terms.
When we combine them, we get: \[ 5z^2 - 5z^2 = 0 \]
Therefore, the expression simplifies to: \[ 6y^3 + 0 = 6y^3 \]
So the final result is: \[ \boxed{6y^3} \]