Help! I'm looking for the second derivative of a parametric equation?

y = 3 cos t
dy/dt = -3 sin t

x = sin t
dx/dt = cos t

dy/dx
= (dy/dt)/(dx/dt)
= - 3 sin t / cos t
= - 3 tan t

d²y/dx²
= (d/dx) (-3 tan t)
= -3 sec² t (dt/dx)
= -3 sec² t / (dx/dt)
= -3 sec² t / cos t
= -3 sec² t / (1/sec t)
= -3 sec² t × sec t
= -3 sec³ t