For this case what you should do is use the following trigonometric relationship:
sin (x) = C.O / h
Where
x: angle
C.O: opposite leg
h: hypotenuse
Substituting the values we have:
sin (60) = long / Hypotenuse
sen (60) = long / 12
long = 12 * sen (60)
long = 10.39
Answer:
long = 10.39
