Assume that the long side is an opposite side. Assume also that the angle 60° is between the opposite side and the hypotenuse.
We are going to cosine function which has a ratio of
cos θ = opposite side / hypotenuse
cos θ = a/c
Solving for the hypotenuse c
c = a/cos 60°
c = 3/cos 60° = 6 (ANSWER)
