What we know is that the sum of a triangle's internal angles are 180º, and a straight angle makes 180º.
first, we have to find the third internal angle(let's call it n) so that we can use it to find the exterior angle.
(3x + 20) + (4x + 5) + n = 180
simplify:
7x + 25 + n = 180
7x + n = 155
n = 155 - 7x
Now we can add it to the external angle to find x.
(155 - 7x) + (8x + 15) = 180
simplify:
170 + 8x - 7x = 180
170 + x = 180
x = 10
Now we can substitute it to the external angle.
8 x 10 + 15 = 95
the exterior angle is 95º.