The quotient of the opposite of a number squared and three

"The quotient of the opposite of a number squared and 3"

Take "the opposite of a number squared" and call it y.

So you get "The quotient of y and 3"

This is y/3.

Now what is y? "The opposite of a number squared"

Take "The opposite of a number" and call it z.

So y is "z squared"

Replacing y, we get z^2 / 3

But what is z? "The opposite of a number"

Call "a number" x.

The opposite of x is - x.

So z is "-x"

Replacing z, we get (-x) ^2 / 3