Hey, here's how you work it out.
The efficiency equation is ( useful output energy / total input energy ) * 100 (for a percentage). It's not direct but I think the "total input energy" would be the EPE and the "useful output energy" would be the amount that was transferred to the toy car's kinetic energy store. You have to work out both.
You have to be careful with units here.
The equation for EPE = 1/2 * k (N/m) * e^2 (m)
k = 100 N/m
You're given e (extension) in cm! You have to convert this to metres to get the right answer, so that's 0.02 m
So you do 1/2 * 100 * 0.02^2 = 0.02 J
Now for kinetic energy:
KE = 1/2 * mass (kg) * velocity^2 (m/s)
You're given the mass as 150g, in kilograms, this would be 0.15 kg.
v = 0.25 m/s
So 1/2 * 0.15 * 0.25^2 = 4.6875 * 10^-3
Now you have both your "useful" and "total inputted" energy, you can work out the efficiency of the toy car.
( 4.6875 * 10^-3 / 0.02 ) * 100 = 23.4 % (3 s.f)