3x + 6 = 24 + (2x +18) because the two interior angles are subtracted from 180 to make the last interior angle, and that very angle also equals 180 minus the exterior angle, which infers that the exterior angle equals the sum of the two interior angles

now you just solve

like this

3x + 6 = 24 + 2x + 18

3x + 6 = 2x + 42

move the same numbers to the same sides

3x - 2x = 42 - 6 (remember that the signs changes if moved through the "=" )