The sum of the angle measures of the exterior angles is 180.
The exterior angle refers to the angle between a side of a polygon and an extended adjacent side.
Now, given that 5x-21 and 2x+12 are two alternate exterior angles. We know that the sum of the two exterior angles is 180° because the two exterior angles are lying on a straight line and straight line is 180°.
Thus, (5x-21)+(2x+12) = 180°
=> 5x-21+2x+12 = 180
=> 5x+ 2x -21 +12 = 180
=> 7x - 9 = 180
=>7x = 189
=> x = 189/7 = 27
Hence the angles are:
And sum = 114+66= 180
To learn more about exterior angles from the given link:
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