在英语中，字母z是最后一个字母。

In English, ___z is the last letter of the alphabet.

在这个方程中，z 代表未知数。

In this equation, ___z represents the unknown quantity.

我昨晚没睡好，今天下午得补个觉。

I didn't sleep well last night, so I need to catch some ___z's this afternoon.