\(\displaystyle 102,91=\frac{6}{1,035}+\frac{6}{(1+r)^{2}}+\frac{106}{1,035 \cdot 1,0576^{2}} \)
\(\displaystyle 102,91-\frac{6}{1,035}-\frac{106}{1,035 \cdot 1,0576^{2}} = \frac{6}{(1+r)^{2}}\)
\(\displaystyle 5,549346679... = \frac{6}{(1+r)^{2}}\)
\(\displaystyle (1+r)^{2}= \frac{6}{5,549346679... }\)
\(\displaystyle 1+r= \sqrt{\frac{6}{5,549346679... }}\)
\(\displaystyle r= \sqrt{\frac{6}{5,549346679... }}-1 \approx 3,98\, \%\)