To find the measure of the third angle in a triangle, we can use the fact that the sum of the interior angles of a triangle is always $180^\circ$.

Given:

• First angle = $88^\circ$
• Second angle = $21^\circ$

Let's calculate the third angle:

$\text{Third angle} = 180^\circ - (88^\circ + 21^\circ) = 180^\circ - 109^\circ = 71^\circ$

So, the measure of the third angle is $71^\circ$.

Would you like more details or have any questions?

Here are some related questions:

1. What is the sum of interior angles for a quadrilateral?
2. Can the third angle in a triangle be obtuse if the other two angles are both acute?
3. How do you determine if a triangle is acute, obtuse, or right based on its angles?
4. What is the measure of the third angle if two angles are both $45^\circ$?
5. Can the angles of a triangle be greater than $90^\circ$?

Tip: The sum of interior angles for any $n$-sided polygon is $(n - 2) \times 180^\circ$.