A 27-year-old woman presents to the emergency department with 8 weeks of amenorrhoea, lower abdominal pain and light vaginal spotting. Urine pregnancy test is positive. She is haemodynamically stable. Transvaginal ultrasound shows no intrauterine gestational sac and a 2.5 cm adnexal mass on the right with moderate free fluid. Serum β-hCG is 3 000 IU/L. What is the most appropriate management?