public String execute(String cmd) {
            List<String> lista = this.getNames();
            String [] exec = cmd.split("\\ ");
            Double z;
            for (String i : lista) {
                if (exec.length == 3 && exec[0].equals(i)) {
                    Double x = Double.parseDouble(exec[1]);
                    Double y = Double.parseDouble(exec[2]);
                    z = Math.pow(x,y);
                    return z.toString();
                }
            }
            throw new IllegalArgumentException();
        }