This question applies the rules of angles on a plane. The transversal that cuts the two parallel lines is a decisive one. From there you can determine which angles are opposite, alternate and so on.
Obeserve carefully that angle 54 and angle a lie on a straight line.
"Angles on a straight line sum up to 180 degrees."
Therefore,
Angle 54 + Angle A = 180
54 + A = 180
Subtract 54 from both sides of the equation
54 - 54 + A = 180 - 54
A = 126
Also note that;
"Opposite angles are equal in size."
Angle B is opposite to angle 54
Therefore angle B is 54 degrees.
Note also that angle C is opposite to angle A, therefore angle C equals angle A and that makes angle C = 126
If the two parallel lines are cut by a transversal, then it makes it easy to identify alternate angles. Alternate angles are formed on the inner sides of the two parallel lines but on the opposites sides of the tranversal. If you observe VERY CLOSELY, it usualltakes the form of a Z shape. You can equally determine alternate angles on the outer parts of the parallel lines in which case it becomes "exterior alternate angles."
"Alternate angles are equal."
Observe carefully and you'll see that angle B and angle D are interior alternate angles. That means B equals to D and therefore angle D = 54 degrees.
Similarly, angle A and angle G are alternate angles. Therefore angle G = 126 degrees.
Also angle F is opposite to angle D, and therefore angle F = 54 degrees.
Angle E is opposite to angle G, therefore angle E = 126 degrees