Answer:
53
Step-by-step explanation:
they are a linear pair so x + 2x + 21 = 180 so you subtract 21 from both sides, combine x and 2x then divide by 3 to get 53
x = 53
Assuming that the river is of constant width throughout.
x + (2x + 21) = 180
x + 2x + 21 = 180
3x + 21 = 180
3x = 159
