Like most solutions to chemistry problems, there is more than one method of reaching the answer.

For example, this question provides the $K$_b of $NH$₃, and thus the easiest solution is to solve for the $[\; OH-]$, rather than the $[\; H+]$. However, solving for $[\; H+]$ is just as valid because the $K$_a for the acid dissociation equation is related to the $K$_b provided by:

Refer to Section 7: Relationship Between $K$_a and $K$_b for further reminders!