I've been working on this differential equation for days now but I can not find a solution. I understand that finding a solution in closed form is difficult to say (assuming there is). What I ask is if there is a substitution (change of variables) that can "transform" this differential equation from the second order to the first order so that it is not too constrained. Thank you for your interest and time.

${y}^{\u2033}(x)+a\text{}x\mathrm{cos}y(x)=0$

where $a\in \mathbb{R}\text{}$

${y}^{\u2033}(x)+a\text{}x\mathrm{cos}y(x)=0$

where $a\in \mathbb{R}\text{}$