$x=x_1+x_2=A\cos(20\pi t+\varphi}$
$A=\sqrt{2^2+4^2+2.2.4\cos(\dfrac{\pi}{3}-\dfrac{\pi}{4})}\approx 6(cm)$
$\tan\varphi=\dfrac{2\sin\dfrac{\pi}{3}+4\sin\dfrac{\pi}{4}}{2\cos\dfrac{\pi}{3}+4\cos\dfrac{\pi}{4}}$
$\Rightarrow \varphi \approx 0,872(rad)$
$\to x=6\cos(20\pi t+0,872)$