Answer
pOH is the negative logarithm of the $OH^-$ concentration in a solution:
$pOH = -log[OH^-]$
$pH + pOH = pKw$
At 25$^{\circ}$
$pH + pOH = 14$
Work Step by Step
You can find this information in any textbook that approach acids and bases, or in the internet.
You can also find this pH-pOH relation using the equilibrium equation of water.
$[H_3O^+]*[OH^-] = K_w$
$-log([H_3O^+]*[OH^-]) = -log(K_w)$
$-log[H_3O^+] - log[OH^-] = pK_w$
$pH + pOH = pK_w$
