I assume a, b, and c are the measures of the angles A, B, and C themselves. (If you meant lengths of the sides opposite these angles, this would be impossible.)
The interior angles to any triangle sum to 180° in measure, so
A + B + C = 180°
Now,
• A is twice the size of B, so A = 2B or B = A/2
• C measures 20° less than A, so C = A - 20°
Substitute these into the first equation and solve for A :
A + A/2 + (A - 20°) = 180°
5/2 A - 20° = 180°
5/2 A = 200°
A = 80°
Solve for B and C :
B = 80°/2
B = 40°
C = 80° - 20°
C = 60°