From $x(0)=x_{0}=c_{1}$ we see that $x(t)=x_{0} \cos \omega t+c_{2} \sin \omega t$ and $x^{\prime}(t)=-x_{0} \sin \omega t+c_{2} \omega \cos \omega t .$ Then $x^{\prime}(0)=x_{1}=c_{2} \omega$ implies $c_{2}=x_{1} / \omega .$ Thus $$x(t)=x_{0} \cos \omega t+\frac{x_{1}}{\omega} \sin \omega t.$$