Answer:
[tex]\frac {f_i}{\sqrt 2}[/tex]
Explanation:
Frequency is given by [tex]\frac {1}{2\pi}\sqrt{{\frac {k}{m}}[/tex]
Let the initial frequency be denoted by [tex]f_i[/tex]
[tex]f_i=\frac {1}{2\pi}\sqrt{{\frac {k}{M}}[/tex]
When M is 2M then
[tex]f_f=\frac {1}{2\pi}\sqrt{{\frac {k}{2M}}=\frac {f_i}{\sqrt 2}[/tex]
Therefore, the final frequency is
[tex]\frac {f_i}{\sqrt 2}[/tex]