12 less than the quotient of 12 and a number z

This expression would be written as 12/z - 12.

When we write expressions of sentences like this, we need to separate out the different terms. Remember than 12 less than would come after, since it is subtraction.

quotient of 12 and a number z ... 12 less than

(12/z) ... - 12

Put that together

12/z - 12