[AWK]

It is not true for weak bases, as in this case.

[AWK]

Read Chembuddy lecture carefully (link is given twice). Check applicability of equations 8.8 and 8.12 to your problem (use K_b instead of K_a and OH^- instead of H⁺).
Calculate c_i for both solution (check also table Results of pH calculation of acetic acid). Then for then mean value of c (=> equal volumes) calculate OH^-, pOH and finally pH.
The final pH should be between values given for you solutions, but very close to one of these values.

[docnet]

Use the Henderson Hasselbalch equation: pOH = -log(K_b) + log (B⁺/BOH) and use ratios to find the pH of the final solution, taking into account that equal volumes of the two solutions are mixed!

You might not this here, but the following equations are useful for pH problems.
[H⁺] = 10^-pH
[OH^-] = 10^-pOH

I hope this helped at least a little bit. Have a nice day!

[AWK]

Use the Henderson Hasselbalch equation

H-H equation is useless for single weak acid or base.