\(\omega_1=\sqrt{\frac{k}{m}}\)
\(\omega_2=\sqrt{\frac{k}{m+2}}\)
\(\frac{\omega_1}{\omega_2}=\sqrt{\frac{m+2}{m}}\)
Since \(\omega=\frac{2 \pi}{T}\)
\(\frac{T_2}{T_1}=\sqrt{\frac{m+2}{m}}\)
\(\left(\frac{5}{4}\right)^2=\frac{m+2}{m}\)
\(\frac{25}{16}=\frac{m+2}{m}\)
\(25 \,m=16 \,m+32\)
\(9 \,m=32\)
\(m=3 \frac{5}{9} \,kg\)
\(m=3.5 \,kg\)