A 29-year-old woman presents with severe right lower quadrant abdominal pain and vaginal spotting. Her last menstrual period was 7 weeks ago. β-hCG is 3000 IU/L. Transvaginal ultrasound shows no intrauterine pregnancy and a complex right adnexal mass with free fluid in the pelvis. She is haemodynamically stable. What is the most appropriate management?