Respuesta :

sqdancefan
The exterior angle is equal to the sum of the opposite interior angles.
   (4x+9) = (2x) + (3x-12)
   4x + 9 = 5x - 12 . . . . . . . . collect terms
   21 = x . . . . . . . . . . . . . . . .add 12-4x

Then the exterior angle is
   (4·21+9)° = 93°