Answer: a = 15
Step-by-step explanation:
The jump from 13 to 39 is "times 3" when comparing the diagonal segments. So the jump from 5 to 'a' must also be "times 3" to keep the sides in the same proportion. Therefore, a = 3*5 = 15.
Or we could solve it like so
13/39 = 5/a
13a = 39*5 ... cross multiplying
13a = 195
a = 195/13 .... dividing both sides by 13
a = 15