Angry Bird: Spring Constant of Slingshot

Using Tracker, I measure the length of the stretch of the slingshot as 2.43 m. 

• Use tracker to find the velocity of the bird just as it is released from the slingshot. 
• If the angry bird has a mass of 135 kg (remember it is a big bird ~0.7m high was the measurement for acceleration to be 9.8 m/s²—the mass of a 70cm diameter spherical water balloon is 180kg—water density is 1000 kg/m³ and average bird density[1] is around 750 kg/m³), what is the kinetic energy of the bird just after release from the slingshot (max kinetic energy)? 
• What is the spring constant of the slingshot (note that the slingshot seems to break soon after launch)?