Alrighty, the sum of all the angles of a quadrilateral, as you might know, is 360 degrees. So since you're given all the 3 angles, you can find the 4th one by subtracting the sum of all the 3 angles from 360.
![360 - (98 + 72 + 105)](https://img.qammunity.org/2021/formulas/history/high-school/b76d7071r6o1ryzfcv7iqo5l5f9lomzfpw.png)
which when evaluated gives,
![360 - 275 = 85](https://img.qammunity.org/2021/formulas/history/high-school/98xr17a1r8x74o8s5q26hofec2tqp9p3n1.png)
Now that's the interior angle not the exterior. To get the exterior we need to subtract the angle from 180 which gives:
![180 - 85](https://img.qammunity.org/2021/formulas/history/high-school/9sebxuqxnuk5ps1fk06tcrk8a2z75twzfa.png)
which evaluates to
.
Here's the diagram if needed :P