It is very important to know that neutral pH changes with temperature. The key part to this problem is that the solution is neutral! In a neutral solution the pH and pOH are equal and therefor so are their concentrations.
Kw= [H]*[OH] where [H] and [OH] represent the concentrations of those ions.
pH=-log[H]
antilog(-pH)=[H]
Antilog is fancy way of saying 10^x. In our case x=-7.64
[H]=2.95 x 10^-8 M (Molarity)
Since pH and pOH are equal in our neutral solution so are their concentrations.
[OH]=2.95 x 10^-8 M
Now we go back to Kw= [H]*[OH] and plug in the concentrations.
Kw=(2.95 x 10^-8)*(2.95 x 10^-8)=8.71 x 10^-16
