As provided in the question, the pH of a solution is equal to the negative logarithm of the H₃O⁺ concentration in the solution (the square brackets denote the molar concentration).
We are given a solution with an H₃O⁺ concentration of 5.6 × 10⁻⁹ M. So, the pH of this solution would be:
pH = -log[H₃O⁺] = -log(5.6 × 10⁻⁹ M) = 8.25.
A pH that is equal to 7 is neutral; less than 7, acidic; and greater than 7, basic. As 8.25 is greater than 7, this solution is basic.