Strong acids react essentially to completion with weak bases. If there is more strong acid than strong base, then the strong acid will react with all available weak base and there would be some 'left over' strong acid. After this, you could treat the 'left over' strong acid as reacting with water (assuming water is your solvent) to make H3O+. If there is much more strong acid than weak base, you could get a reasonable estimate of the pH by only considering the pH from the 'left over' strong acid that reacts with water. If there is only slightly more strong acid than weak base, you might need to consider the pH effects from both the 'left over' strong acid reacting with water and the weak conjugate acid formed by reacting weak base with strong acid.
CH3NH3+ is a weak acid and HCl is a strong acid. In this question your are asked about CH3NH2 (ie the conjugate base of CH3NH3+), which is a weak base, and HCl, which is a strong acid. Technically CH3NH2 could act as either an acid or a base, but when mixed with HCl (a strong acid) it will act as a base (since it is a much much weaker acid than HCl).
