The given picture could be seen as follows
The way to give the pair 3 and 5 a name is based on their location regarding the lines that form the angles. Note that the lines l2 and l3 are parallel and that angles 3 and 5 are in between this two lines. Hence, they receive the name of interior angles.
Note also that angles 3 and 5 are on different sides of the line that crosses lines l2 and l3. Hence they are alternate angles.
Combining this two results, the angles 3 and 5 are alternate interior angles
