How do you calculate a DC offset from RMS and peak to peak voltage
Summary:


RMS powers can be added.
So - RMS Volts^2 can be added. 
Offset = Vdc = sqrt( Vrms_total_signal^2 - Vrms_ac_only^2)  

= sqrt (5.452^2 - 5.175^2) = 1.716 volt 


(Voltages above are mean RMS voltages from supplied table. ) 





Calculating AC only RMS from Vpk-pk would give


Vrms_AC = (Vpk-pk)/2 x 1/sqrt(2) = Vpk-pk/2.sqrt(2) 

= 14.68/2/1.4142 = 5.190 V RMS.


The RMS voltage displayed is 5.175V mean suggesting that
 the signal is not quite a pure sinewave
 and/or that the measurement system is not 100.000% accurate (of course).   

5.19 / 5.175 = 1.0029 or about 0.3% higher.



"RMS powers" add directly.
 ie Power total = Power_ACV + Power_DC
 Power is proportional to V^2 so
 (Vrms_AC+DC)^2 = (Vrms_AC)^2 + (Vrms_DC)^2

as Vrms_DC = Vdc

Vrms_combined^2 = Vrms_AC^2 + Vdc^2

or Vdc^2 = Vrms_combined^2 - Vrnms_ac^2

So Voffset = Vdc = (Vrms_combined^2 - Vrma_ac^2)^ 0.5

Here
 Vrms_ac = 5.175V average (from table)
 Vrms_combined = 5.452 mean
 So
 Voffset = sqrt( 5.453^2 - 5.175^2) = 1.716 Volt.