How do I solve this AP Chemistry problem?

Question

concentrated hydrochloric acid and 20.0 mL concentrated nitric acid to 300 mL water. More water is added until the final volume is 1.00 L. Calculate [H+], [OH-], and the pH for this solution. Hint: Concentrated HCL is 38% HCL (by mass) and has a density of 1.19 g/mL; concentrated HNO3 is 70% HNO3 (by mass) and has a density of 1.42 g/mL.

Answer 1

1) for each ingredient, initial amount added of the active substance = concentration x density x volume taken.

2) From amount, use MW of each to calculate #mol used of each.

3) Both HNO3 and HCl are strong acids => all their protons are released.

4) [OH(-)][H(+)]=10^-14