Hello Spongebob :-)
To find the missing side, you have to write a proportion because the sides are proportional.
When an angle is bisected like this, you can write:
28/x = 12/(30 - x)
Cross multiplying will give you:
12x = 840 - 28x
40x = 840
x = 21
You could plug the 21 back in and you will see that the sides are proportional.
Good luck!