How does extracellular potassium ion concentration and calcium ion concentration affect the excitability of a cell?
If extracellular $Ca^{2+}$ concentration increased, it makes the extracellular membrane more positive then it used to be, then it would take the membrane potential to lower level then the excitability of the cells will be decreased

if extracellular $K^+$ increased more than the concentration inside the cell than it will make the $K^+$ on extracellular space move inside the call down the concentration gradient, it would take the potential membrane to higher state make it easier to reach the threshold (easier to be excitable)