40°+B+C = 180° (sum of angles in a triangle)
sub. in values of B and C :
40° + (2x-30)° + (x+20)° = 180°
expand the equation and solve for x:
2x + x + 40° - 30° + 20° = 180°
3x + 30° = 180°
3x = 150°
x = 50°
sub. x = 50° into B,
2(50°) - 30° = 100° - 30°
= 70°
B = 70°
