In the reality television show "Amazing Race", a contestant is firing 12-kg watermelons from a slingshot to hit targets down the field. The slingshot is pulled back 1.5 m and the watermelon is considered to be at ground level, 0.3 m below the launch point. The targets are at ground level 12 m horizontally away from the launch point. Calculate the spring constant of the slingshot (in N/m). (Assume the angle that the watermelon's velocity makes with the horizontal at the launch point is the same as the angle the slingshot makes with the horizontal when pulled back. Also assume the equilibrium length of the slingshot is negligible.)