Using the second equation, plug in t = 0 and you will get y = 9.
So when you differentiate implicitly for the first equation, you will have some x's, t's, and dx/dt's. Just solve for dx/dt and don't plug in values yet.
Then for the second equation you will have y's, t's, and dy/dt's. Solve for dy/dt without plugging in values.
Then take dy/dt divided by dx/dt to get dy/dx.
Then plug in x = 0, y = 9, and t = 0 and you should get the proper answer.