Answer:
1820.44 N/m
Step-by-step explanation:
mass of ball, m = 4 kg 
velocity of ball, v = 16 m/s 
distance x = 0.75 m 
let k be the spring constant. 
By the use of conservation of energy, 
the kinetic energy of the ball is equal to the potential energy of the spring 

4 x 16 x 16 = K x 0.75 x 0.75 
K = 1820.44 N/m 
Thus, the spring constant is 1820.44 N/m.