This can be solved by equating the future value of the payment F1, and the future value of the annuity F2, after n=12*15=180 months.
i is the monthly interest.
P=payment of 90000
A=monthly amount of 850
F1=P(1+i)^n=90000(1+i)^180
F2=A*((1+i)^n-1)/i=850((1+i)^180-1)/i
equate F1=F2 and solve for i (only unknown) by trial and error, fix-point iteration or Newton's method to get i=0.00650439,or 0.650439%
The monthly interest rate is 0.650439%.
Therefore the APR=12*i=7.805271%, or the effective interest rate is
(1+i)^12-1=8.09064%