Explanation:
straight line = 180 degree
example
1) 2w - 24 + 2w - 40 = 180
4w = 180 + 24 + 40
4w = 244
w = 244/4
.......
2) 13n - 11 +(-9 + 12n)
13n - 11 -9 + 12n = 180
25n -20 = 180
25n = 180 + 20
25n = 200
n = 200/25
.........
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