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+### Statement
+
+$2.3.9.$ [Insert the problem statement]
+
+### Solution
+
+### Statement
+
+$2.3.9.$ A slingshot was made from a long rubber band of stiffness $k$. Find the kinetic energy of a "projectile" fired from this slingshot if the rubber band was stretched with a force $F$ and then released.
+
+### Solution
+
+When the slingshot is stretched by a distance $x$ from both ends, the total pulling force $F$ is balanced by the tension in the two halves of the rubber band. Let's write the equilibrium condition:
+$$\frac{F}{2} = kx$$
+Expressing the elongation $x$ of the rubber band:
+$$x = \frac{F}{2k} \quad (a)$$
+
+The potential energy of the stretched rubber band at this point is:
+$$E_{\text{pot}} = \frac{kx^2}{2}$$
+Substituting expression (a) into the energy formula:
+$$E_{\text{pot}} = \frac{kF^2}{8k^2} = \frac{F^2}{8k}$$
+
+From the law of conservation of energy, all the potential energy of the elastic band $E_{\text{pot}}$ converts into the kinetic energy of the projectile $E_k$:
+$$E_{\text{pot}} = E_k \implies E_k = \frac{F^2}{8k}$$
+
+#### Answer
+$$E_k = \frac{F^2}{8k}$$
+
+#### Answer
+
+[Insert a concise answer or boxed result]