Greenland is x
Iceland is g
g + x = 880,000
x = 21g
Solve both equations for x (Greenland)
x = 880,000 - g
x = 21g
Because of the transitive property, which states that for any real numbers a, b, and c: If a = b and b = c, then a = c, you can set the two things that equal x equal to each other.
21g = 880,000 - g
Then you simplify.
21g = 880,000 - g
21g + g = 880,000 -g +g
22g = 880,000