Position of Cart attached to Wall by Spring

Problem Definition
Then the horizontal position of $C$ at time $t$ can be expressed as:
 * $x = C_1 \cos \alpha t + C_2 \sin \alpha t$

where:
 * $C_1$ and $C_2$ depend upon the conditions of $C$ at time $t = 0$
 * $\alpha = \sqrt {\dfrac k m}$

Proof
From Motion of Cart attached to Wall by Spring, the horizontal position of $C$ is given as:
 * $\dfrac {\mathrm d^2 \mathbf x} {\mathrm d t^2} + \dfrac k m \mathbf x = 0$

As both $k$ and $m$ are positive, $\dfrac k m$ can be expressed as:
 * $\dfrac k m = \alpha^2$

for some $\alpha \in \R_{>0}$.

Hence:
 * $\dfrac {\mathrm d^2 x} {\mathrm d t^2} + \alpha^2 x = 0$

and so from Second Order ODE: $y'' + k^2 y = 0$:
 * $x = C_1 \cos \alpha t + C_2 \sin \alpha t$

Hence the result.