7-24

$${W_e = \int_{x_i}^{x_f} -kx\, dx = -k \int_{x_i}^{x_f} x\, dx \\ = (- \frac{1}{2}k)[x^2]_{x_i}^{x_f} = (- \frac{1}{2}k) (x_f^2 - x_i^2)}$$