When the two angles are next to each other on a straight line, they will add up to 180 degrees.
So, we can add up the expressions and set them equal to 180 degrees.
16x+3 + 12x+9 = 180
Combine like terms:
28x = 168
x= 6
Now we know what x is, all we have to do is sub in to the original expressions to get the value of the angles.
16(6) + 3 = 99
12(6) + 9 = 81
The left angle is 99 degrees, and the right is 81 degrees. You can test this again by adding it up and making sure they add to 180.
Hope this helps!