A spring that obeys Hooke's Law (F(x) = kx) has length 12" at rest. A force of 15 lbs. compresses the spring to 11"

a) What is the vale of the spring constant k?

b) How much work is done to stretch the spring from 12" to 15"?

I think a) is 15/11.

and on b) i got this far.

(15/11) Integral of 3x on the intervals [12,15]

anybody know what i am talking about?