A 30-year-old woman presents with 6 weeks of amenorrhoea and right iliac fossa pain. She has light vaginal spotting. Serum β-hCG is 2,000 IU/L. Transvaginal ultrasound shows no intrauterine gestational sac and a 2.5 cm right adnexal mass with free fluid in the pouch of Douglas. She is haemodynamically stable. What is the most appropriate management?