The question is for skeletal muscle; RMP = - 90 mV. When Na+, K+and Cl- are considered to diffuse together and reach equilibrium, the membrane potential at which all of them reach equilibrium can be calculated by Goldman's constant field equation. The equation takes into account the concentration gradients of the respective ions, relative permeability of the membrane for the ions and polarity of the electrical charge. When all the 3 ions reach equilibrium, charge on the membrane would be- 86 mV.An additional - 4 mV is contributed by Na+/K+-pump.