Once again, this looks like a plug and play for the Nernst equation

#DeltaPsi_(K^(+)) = (RT)/(zF) * ln(([K^(+)]_"out")/([K^(+)]_text(in)))#, where

#R# - the universal gas constant; #z# - the valence of the ion - in your case, this will be +1; #F# - Faraday's constant; #[K^(+)]_"out"# - the concentration of the potassium cations outside the cell (in the extracellular liquid);

#[K^(+)]_"in"# - the concentration of the potassium cations inside the cell (intracellular liquid).

Replace the natural log with the base 10 log to get

Since you're not working at #37^@"C"#, you can't use the 61.55 mV value for that product of constants. However, that will not be a problem since you've got all the values you need to solve for #[K^(+)]_"out"#.