G)
Angles on a straight line add to 180°;
According to this rule, we can formulate the following equation:
(2x + 4) + (x + 8) = 180
Now, just simplify and solve for x:
3x + 12 = 180
3x = 168
x = 56°
H)
What is shown on the diagram are sometimes called C-angles;
They also add to 180°;
This C-angle rule is one of 3 that exist when you have a pair of parallel lines (in this case the two horizontal ones) and one line passing through them both.
Using the C-angle rule, we can formulate the equation:
(3x + 9) + (2x - 4) = 180
Simplify and solve:
5x + 5 = 180
5x = 175
x = 35°