Solution
= Solution
Each spring <force> is its extension multiplied by the <spring constant>, acting to restore the <equilibrium>. The <equations of motion> are
$$
\boxed{m\ddot x_1=k(x_2-x_1),\qquad M\ddot x_2=k(x_1-2x_2+x_3),\qquad m\ddot x_3=k(x_2-x_3).}
$$
No end is attached to a wall. The spring extensions are $x_2-x_1$ and $x_3-x_2$, giving <potential energy> $k[(x_2-x_1)^2+(x_3-x_2)^2]/2$.