3sinA = 5sinB (this is known)
According to sine theorem
∴3a=5b,b=3/5a
b+c=2a
∴3/5a+c=2a
∴c=7/5a
According to cosine theorem:
cosC=(a？ +b？ -c？ )/(2ab)
=(a？ +9/25a？ -49/25a？ )/(2*a*3/5a)
=(- 15)/30
=- 1/2
∴C= 120？
Option b
I don't understand, I hope the landlord will adopt it