Question

What is the pOH of a solution hydrogen ion concentration is 5.00×10^-4m?

Answer 1

I assume you meant 5.00E-4 M.

M stands for molarity.
m stands for molality. They aren't the same.
pH = -log(H+) = -log(5.00E-4) = ?
Then pH + pOH = pKw = 14. You have pKw and pH, solve for pOH.