To solve problems like these, we simply need to combine our like-terms as we would with normal operations.
Like-terms are a pair of terms that have the same variables and exponents, and can be added together.
In this case, "4z²" and "z²" are like terms. We can look at "z²" as "1z²" for adding purposes.
Using the logic above, we can see that the simplest form of this equation is 5z²+6a.