Question

Anybody know the answer?

Answer 1

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