$$y'(t)+7\sin(t)y(t)=(te^{\cos(t)})^7$$ $$y'(t)e^{-7\cos(t)}+7\sin(t)y(t)e^{-7\cos(t)}=t^7$$
Rewrite it as $$\implies (y(t)e^{-7\cos(t)})'=t^7$$ And integrate...
$$y'(t)+7\sin(t)y(t)=(te^{\cos(t)})^7$$ $$y'(t)e^{-7\cos(t)}+7\sin(t)y(t)e^{-7\cos(t)}=t^7$$
Rewrite it as $$\implies (y(t)e^{-7\cos(t)})'=t^7$$ And integrate...