Great first extension!
Have you considered having only one method that returns a List of both solutions of the equation?
You could do something like:
public YailList Calc(float a, float b, float c) {
float x1 = /*YOUR CODE TO FIND THE FIRST SOLUTION*/;
float x2 = /*YOUR CODE TO FIND THE SECOND SOLUTION*/;
return YailList.makeList(new Object[] {x1, x2});
}