A 1670 kg car rolling on a horizontal surface has a speed of 80 km/hr when it strikes a horizontal coiled spring and is brought to rest in a distance of 6.5 m. What is the spring constant (in N/m) of the spring? (Ignore nonconservative forces such as friction.)

Now I've tried using the formula 1/2kx^2 but its not working where am I going wrong?

Wc = M*g = 1670 * 9.8 = 16,366 N.

N/m = 16,366N./6.5m.