Hi, I'm in AP Calculus BC, but I'm on medical leave without a teacher or tutor, so I'm stuck on a couple problems. Please show me how to do these, it would be greatly appreciated!

1. The motion of a particle is given by x=-2t^{2 }and y=t^{3}-3t+9, t>0. Find the coordinates of the particle when its instantaneous direction of motion is horizontal.

I know the answer is (-2, 7) but I'm not quite sure how to get there. I have started by finding dx/dt= -4t and dx/dy= 3t^{2}-3, but unsure of where to go from here.

2. The motion of a particle is given by x=lntand y=t^{2}-4t. Find the coordinates of the particle when its instantaneous direction of motion is horizontal.

I know the answer is (ln2, -4) but I'm not quite sure how to get there. I have started by finding dx/dt= 1/t and dy/dt= 2t-4, but unsure of where to go from here.

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