Suppose z=a+bi, where a and b are integers and i is the imaginary unit. We are given that |1+iz|=|1−iz| and |z−(13+15i)|<17. Find the largest possible value of a+b.

Details and assumptions
i is the imaginary unit, where i^2=−1.

I got it till now

maximize a+b, if Re (b)=-Im(a), Re (a)-21 <Im (b)<Re (a)-5
So find max a+b