Calculate [H3O+] for this solution: pH = 8.85? Can you please show how the answer is ##1.4×10^-9M##?

This is a fairly straightforward problem, all you need to know is that

##[H_3^+O] = 10^(-pH)##

Plugging in the value will produce

##[H_3^+O] = 10^(-8.85) = 0.00000000141 = 1.4 * 10^(-9) M##

SInce the pH is greater than 7, you are dealing with a basic solution

##pOH = 14 - pH = 14-8.85 = 5.15##

This means the concentration of ##OH^-## is

##[OH^-] = 10^(-pOH) = 10 ^-5.51 = 7.1 * 10^-6 M##