Sure. We see all three right triangles are similar since each has corresponding congruent angles. We can think of them as all having a common three way ratio:
short leg : long leg : hypotenuse
In the smallest triangle the short leg is a+5 and the long leg is 12.
In the medium triangle the short leg is 12 and the long leg is 18
So the relevant ratio is
(a+5) / 12 = 12 / 18
a+5 = 12 × 2/3
a = 8 - 5
a = 3
Answer: A. 3