Please help me with my chem homework.?

15.0 mL of a buffer is made by mixing 10.0 mL of 0.10 M HF and 5.0 mL of 0.10 M NaOH solution. What is the pH of this buffer after 0.42 mL of 10.0 M hydrochloric acid are added to it? The Ka for HF is 6.8 x 10-4.