Operacion: -x
Traceback (most recent call last):
  File "Calculadora.py", line 229, in <module>
    Calculadora(cuenta)
  File "Calculadora.py", line 66, in __init__
    print self.reducir_polinomios(operacion)
  File "Calculadora.py", line 189, in reducir_polinomios
    res = str(eval(resultado)) + 'x'
  File "<string>", line 1
    -
    ^
SyntaxError: unexpected EOF while parsing