The pH would be 3.16 because the formula for finding the pH from a H± ion is pH = -log [H±]
So with that formula, you first have to substitute the H± with your numbers which in this case is 6.9 × 10^-4.
It should like this:
pH = -log (6.9 × 10^-4)
And lastly, just calculate.
pH = 3.16
Hope that helps, do let me know if you need more help in understanding this!
