Linear equations in one variables shows always graph in straight line,
For ex: x = 3, y = 4, x = -1.5, y = -2/3
Also two variables linear equations with degree of 0 or 1 (Each term) shows always graph in straight line,
For ex: x + y = 5, 2x - 4y = 7 etc.
So you question is most likely about a triangle DEF with the 3 angles being named D, E, F. The triangle would look something like the image attached below.
If that is what is being asked then i need more info to answer but
By Definition it would be 180 degrees.
Here is the answer but basically the work you just plug in 6 for A, 12 for B, and 5 for C..into the quadratic formula and solve from there