Quadratic equation in Lisp

Example for versions Clojure 1.0.0, Clojure 1.1.0 
(defn solve-quadratic [a b c]
  (if (= a 0)
    "Not a quadratic equation."
    (let [D (- (* b b) (* 4 a c))
          k1 (- 0 b)
          k2 (* 2 a)]
         (if (= D 0)
            (str "x = " (/ k1 k2))
            (if (> D 0)
                (let [k3 (Math/sqrt D)]
                     (str (str "x1 = " (/ (+ k1 k3) k2) (str "\nx2 = " (/ (- k1 k3) k2)))))
                (let [k3 (Math/sqrt (- 0 D))]
                     (str (str (str (str "x1 = (" (/ k1 k2)) (str ", " (/ k3 k2))) ")\nx2 = (") (str (str (/ k1 k2) ", ") (str (- 0 (/ k3 k2)) ")") ))
                ))))))

(import '(java.util Scanner))
(def scan (Scanner. *in*))
(def a (.nextInt scan))
(def b (.nextInt scan))
(def c (.nextInt scan))
(println (solve-quadratic a b c))