Answer:
We have a triangle rectangle here.
We know that:
AC = 15
BC = 8
AB = 17
We know that in a triangle rectangle the longest side is the hypotenuse.
In the image we can see that the shorter side is the base, then BC is the base.
AC is the height.
AB is the hypotenuse.
Such that:
A is the top vertex.
B is the bottom right vertex
C is the bottom left vertex.
Now you need to remember the relations:
Sin(θ) = (opposite cathetus)/(hypotenuse)
Cos(θ) = (adjacent cathetus)/(hypotenuse)
Tan(θ) = (opposite cathetus)/(adjacent cathetus)
then:
Sin (A) = (AC)/(AB) = 15/17 = 0.88
Cos(B) = (AC)/(AB) = 15/17 = 0.88
Tan(B) = (AC)/(CB) = 15/8 = 1.875
Now we want to find the measure of angle A.
We know that:
Sin(A) = 0.88
Then if we apply the Asin( ) function in both sides, we get:
Asin( Sin(A)) = Asin(0.88)
A = Asin(0.88) = 61.6°
(the Asin() function is the inverse of the sin() function)
Now we have:
sin(x) = 0.906
We do the same thing as before.
Asin(sin(x)) = Asin(0.906)
x = Asin(0.906) = 65°
And:
Sin(87°) = 0.999
= 2 x 
x 6 = 12