Hi there!
Using the formula - n-2*180 -, we know that the interior angles of a pentagon add up to 540. We can use the values we're given and add them together and set them all equal to 540, then solve.
WORK:
540 = 204 + 12x
336 = 12x
x = 28
Hope this helps!! :)
If there's anything else that I can help you with, please let me know!