The pH of a 1.0 × 10^–8 M H₃O⁺ solution is ____?

Here, we are dealing with ammonium ion
This means the H⁺ concentration is 1.0 × 10^–8 M

Whenever you see a lowercase p, it means 'take the negative log of'
pH means take the negative log of [H⁺], the proton concentration

So let's do just that

pH = -log [H⁺]
pH = -log [1.0 × 10^–8]
pH = -(-7.09)
pH = 7.09

The pH of 0.010 M ammonium ion solution is 7.09