The formula for interior angle of any polygon is (n-2) x 180 / n where n is the number of sides.
An octagon has 8 sides:
Part A.
interior angle = (8-2) x 180 /8
Interior angle = 135 degrees
Part B.
3x -57 = 135
Add 57 to both sides:
3x = 192
Divide both sides by 3:
X = 64
