Once you add indentation, and know the simple rule that args line up vertically (unless they're so short that you'd rather leave them), the following is pretty easy to read:

    (/ (+ (- b)
          (sqrt (- (* b b)
                   (* 4 a c))))
       (* 2 a))

It tells me:

* there's a quotient of 2 things

* the first thing is a sum of -b and a sqrt

* the second thing is a product

and so on. Pretty nice. Of course, mathematical notation is more terse.