Line: 2, Column: 4 Method does not exist or incorrect signature: void sendMail(String, String, String) from the type EmailMissionSpecialist

EmailMissionSpecialist em = new EmailMissionSpecialist();
em.sendMail('Enter your email address', 'Flight Path Change', 
   'Mission Control 123: Your flight path has been changed to avoid collision '
   + 'with asteroid 2014 QO441.');

Try this with your email.