Question 1009952
You can take the description, assign variables, and form two equations.  Are you then forgetting that ONE MORE EQUATION is the sum of the angles being 180 degrees?  Is this enough for you to continue and finish?


Let measures of angles 1, 2, 3, be  x, y, z.

The description first gives you  {{{system(x+y=78+z,y-z=2x)}}}.


When simplifying and using the 180 degree sum,
the system to solve will be {{{system(x+y-z=78,2x-y+z=0,x+y+z=180)}}}.
The rest is yours to do.  You will maybe use Cramer's Rule or some other linear algebra row-reduction, or something.



RESULTS:
x=126,y=53,z=1