[ubcdumbass]

Having trouble with this question. Not asking for the answer ofc, just a step in the right direction.

Q: A cation-sensitive electrode is used to determine the activity of calcium in the presence of sodium. The potential of the electrode in 0.0100 M CaCl2 (aq) measured against an SCE is +195.5 mV. In a solution containing 0.0100 M CaCl2 (aq) and 0.0100 M NaCl, the potential is 201.8 mV. What is the activity of calcium ion in an unknown solution if the potential of the electrode is 215.6 mV versus SCE and the sodium ion activity has been determined with a sodium ion-selective electrode to be 0.0120 M? Assume Nernstian response.

Hint 1: Although we often work in terms of concentrationas an approximation, this question is specifically asking for the activity. You may use the Debye-Hückel limiting law to determine activity coefficients.

Hint 2: Any dissolved ions contribute to the ionic strength, even if the analyte ion.

I'm not sure how to even start. I can't even get the example electrodes to make sense. This is what I can do so far: https://imgur.com/a/qhawdKE
I know how to find ionic strength, nernstain equation and the debye-Huckle limiting law

[Borek]

What does the "Nernstian response mean"?

[ubcdumbass]

I believe my professor meant using the Nerstein equation.
He doesn't have hours to meet to ask or anything so I'm just trying the best I can.