Buffer Question ?

1.
A buffer can be made by mixing a solution of a weak acid and that of its salt (its conjugate base).
The given mixture is not a buffer because HCl is a strong acid, but not a weak acid.

2.
Henderson-Hasselbalch equation:
pH = pKa + log([F⁻]/[HF])
pH = -log(Ka) + log([NaF]/[HF])
pH = -log(3.5 × 10⁻⁴) + log(0.35/0.25)
pH = 3.6