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Answer:
66 degrees
Step-by-step explanation:
The value of an external angle is the sum of the other two angles of the triangle of which that external triangle is sharing with.
This means that angle 4, or 123 is equal to angle 1 plus angle 3, which is 57 degrees.
We have this equation:
, where x is the value of angle 1.
Subtracting 57 from both sides, we have