c) 42°
exterior angle = sum of its opposite interior angles
Which means :-
[tex]x + 21 + 63 = 3x[/tex]
[tex]21 + 63 = 3x - x[/tex]
[tex]21 + 63 = 2x[/tex]
[tex]84 = 2x[/tex]
[tex]2x = 84[/tex]
[tex]x = 84 \div 2[/tex]
[tex]x = 42°[/tex]
Therefore , the correct option is :-
c) 42°
