Question

find the measure of the missing angle.

Explanation:

Step1: Recall angle - sum property

The sum of angles in a right - angled situation (where one angle is $90^{\circ}$) is $180^{\circ}$. Here, we have a right - angle and a $51^{\circ}$ angle along with the unknown angle $a$. The sum of the three angles formed on a straight - line related to this setup is $180^{\circ}$, and since one part is a right - angle ($90^{\circ}$), we consider the sum of the non - right angles. Let the missing angle be $a$. The sum of the two non - right angles is $90^{\circ}$.

Step2: Calculate the missing angle

We know that $a + 51^{\circ}=90^{\circ}$. So, $a=90^{\circ}-51^{\circ}$.
$a = 39^{\circ}$

Answer:

$39^{\circ}$