Well. If you multiply the first one by 2/2, it becomes 2x/6. Then, the two fractions have equivalent denominators. Then, you add them together. This results in an answer or 6x/6. After that, the two 6's cancel out to give you x.
x/3+4x/6
2x/6+4x/6
6x/6
x/1
X is the answer. :D