By the law of cosines, the angle

satisfies

which reduces to

which converts to approximately 77 degrees (this can be done my multiplying by 180 and dividing by pi).
Answer:
18.54
Step-by-step explanation:
sine is opposite/hypotenuse, and you have opposite (b=15), so use that
sin(54) = 15/c
c = 15/sin54
then use calculator
c = 18.54
Each balloon string is 6ft , 6ft translates to two yards he’ll need 114 yards.