if you are comfortable with returning tuples, perhaps the inputs should be a tuple as well...

     def findRoot(p:(Double,Double,Double)) = {
       assert(p._1 != 0)
       val n = sqrt(p._2*p._2-4*p._1*p._3)
       val d = 2*p._1
       ( (-p._2+n)/d, (-p._2-n)/d )
     }
      ----
      scala> findRoot((6,-6,-36))
      res0: (Double, Double) = (3.0,-2.0)