You must find the spring constant and integrate, not just multiply 7 times 4.
A force of 7 lb is required to hold a spring stretched 8 in. beyond its natural length. How much work W is done in stretching it from its natural length to 12 in. beyond its natural length?
Now I thought work was just force times distance. So 7 times 4...but my answer is wrong. What's the deal?