<u>Let's take this problem step-by-step</u>:
<u>The ;unlabeled angle' adjacent to the 'outside angle of measure 78°'</u>
⇒ is on a straight line
⇒ sum of angle measure = 180 degrees
<u>Now we know:</u>
⇒ sum of all angles in a triangle ⇒ 180°
<u>Let's put that in equation form and solve:</u>
<u>Answer: 21°</u>
<u></u>
Hope that helps!
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