1. you are missing the solution values from $\displaystyle 2\pi$ to $\displaystyle 4\pi$
2. $\displaystyle -\pi$ , not $\displaystyle \pi$
3. $\displaystyle \frac{\pi}{2}$ is correct, $\displaystyle \frac{\pi}{4}$ is not
4. is the equation ...
$\displaystyle 2\cos\left(\frac{3}{10x}\right) - 3\sin\left(\frac{1}{2x}\right) = 0 $
???