Question

A tributary joins a river at an angle. Find the value of x.

Answer 1

The sum of the two angles should be 180 degrees because the river is a line.
So, x + 2x +21 = 180
3x + 21 = 180
3x = 159
x = 53 degree

Answer 2

Well you know x + 2x + 21 = 180 degrees
so 3x + 21 = 180
subtract 21 on both sides
3x = 159
divide 3 on both sides
x = 53