Let's take this problem step-by-step:
The ;unlabeled angle' adjacent to the 'outside angle of measure 78°'
⇒ is on a straight line
⇒ sum of angle measure = 180 degrees
[tex]unlabeled_.angle+78 = 180\\unlabeled_.angle = 102[/tex]
Now we know:
⇒ sum of all angles in a triangle ⇒ 180°
Let's put that in equation form and solve:
[tex]x + 102 + 57=180\\x + 159=180\\x = 21[/tex]
Answer: 21°
Hope that helps!
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