We have a review in math but I forgot how to do this. Please explain?

The key hint comes from the zero. It could be either the Additive identity of zero or the Multiplicative identity of zero.

Since we have a + , i'll go for additive identity. The property simply says that if you add zero to anything, your result is what you started with. Since we have z- 2 as a result, and this is what was added to zero, this reinforces our conclusion.