Find Vref of the level shifter when a offset is removed
When it comes to op-amps, think "virtual ground". You want to have the same voltage at both terminals.

If you have \$V_{ref}\$ at the \$+\$ terminal, you need \$V_{ref}\$ at the \$-\$ terminal. Knowing that, you can work out the current flowing through \$R_f\$ in terms of \$V_{out}\$ and \$V_{ref}\$. You can also work out the current flowing through \$R_s\$ in terms of \$V_{sensor}\$ and \$V_{ref}\$.

Additionally for an ideal op-amp, no current flows into the \$+\$ or \$-\$. 

That means any current flowing through \$R_f\$ must be equal but opposite to the current flowing through \$R_s\$. Knowing that, simply equate your two equations for \$I\$ and rearrange to get an equation for \$V_{ref}\$.