Q87. For $n \in \mathrm {~N}$, if $\cot ^ { - 1 } 3 + \cot ^ { - 1 } 4 + \cot ^ { - 1 } 5 + \cot ^ { - 1 } n = \frac { \pi } { 4 }$, then $n$ is equal to $\_\_\_\_$\\