To solve the problem you need to use the following formulas to determine the pH and the pOH.
1) pH=−log[H3O+]
2) pOH=−log[OH−]
3) 14.00=pH+pOH
Given that the pOH is 6.840 the pH will be
pH=14.00-6.840=7.160
To determine the concentrations we need to solve equations 1) and 2). This gives us
[tex][H_3O^+]=10^{-pH}[/tex][tex][OH^-]=10^{-pOH}[/tex]Solving using the pH and pOH values
[tex][H_3O^+]=10^{-7.16}=6.9183\times10^{-8}\text{ M}[/tex][tex][OH^-]=10^{-6.84}=1.4454\text{ }\times10^{-7}\text{ M}[/tex]