Let's say that x is the smallest angle.
This equation could be used to solve for the smallest angle:
x + 2x + 6x = 180
There a 9x total in the equation equaling 180.
Divide 180 by 9 to get 20, which is the measure of the smallest angle.
One angle is twice that, which is 40.
The biggest angle is 6 times that, which is 120.
The three angles are 40º, 20º, and 120º.
Hope this helps!