Find the missing value of a right triangle
a=
b=12
c=15

In a right triangle, angle must obeys Pythagoras theorem:
c² = a² + b²
15² = a² + 12²
225 = a² + 144
a² = 225 - 144
a² = 81
a = √81
a = 9

In short, Your Answer would be 9

Hope this helps!

You would need to use the Pythagorean theorem to solve this. 
There is one equation that you use to find the hypotenuse or c 
c=√a²+b²
Since we already have c we need to use the equation to find a leg 
a=√c²-b²
Now we just plug in all the information we know
a=√15²-12²
a=√225-144
a=√81
a=9