The vector a depends on a parameter t, i.e. $\displaystyle a=a(t)=a_x(t)i +a_y(t)j +a_z(t)k$.. it satisfies the equation $\displaystyle da/dt= j (Vector product) a$

show that $\displaystyle d^2a_x/dt^2 =-a_x$ , $\displaystyle da_y/dt=0$ and $\displaystyle d^2a_z/dt^2 =-a_z$.

For the vector a, find its value for t=pi if at t=0 $\displaystyle a(0)=i+j$ and $\displaystyle da/dt(0)=0k $

i have absolutely no idea how to start...