The rms potential difference is defined as the peak value of the potential difference divided by the square root of 2:
[tex]V_{rms} = \frac{V_0}{ \sqrt{2} } [/tex]
where
[tex]V_0[/tex] is the peak value of the voltage (the maximum voltage).
The generator in our problem has a maximum voltage of [tex]V_0=220 V[/tex], so its rms potential difference is
[tex]V_{rms}= \frac{V_0}{ \sqrt{2} }=155.6 V [/tex]
