z1=3-i
z2=2+3i
z1*z2/z1

(3-i)*(2+3i)/3-i=6+9i-2i-3i^2/3-i=6+7i+3/3-i=9+7i/3-i=(9+7i)*(3+i)/(3-i)*(3+i)=27+9i+21i-7/9-(-1)=20+30i/10=2+3i