Why am I getting the error: cond: expected a clause with a question and an answer

Why am I getting the error: cond: expected a clause with a question and an answer, but found a clause with only one part? What am I doing wrong?

Basically, I have to find out if the discriminant is a positive / negative number.

(define (has-real-roots? a b c)
  (cond
    [(positive? (- (square b) (* 4 a c)))]
    [else false]))