The pH of blood is normally 7.400
What is the hydronium ion concentration?

Whenever you see a lowercase p, it means 'take the negative log of'
pH means take the negative log of [H⁺], the proton concentration
also referred to as the hydronium ion concentration [H₃O⁺]

proton concentration [H⁺] = [H₃O⁺] hydronium ion concentration

So let's do just that

pH = -log [H⁺]
7.4 = -log [H⁺]
-7.4 = log [H⁺]
10^-7.4 = 10^{log [H+]}
10^-7.4 = [H⁺]
[H⁺] = 3.98 x 10^-8

The hydronium ion concentration is 3.98 x 10^-8