seemingly easy curvilinear motion problem

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The box of negligible size is sliding down along a curved frictionless path defined by the parabola y=0.4x^2. When it is at A=(2,1.6) the speed is va=8(m/s)(downward) and the increase in speed is dva/dt=4(m/s^2). Determine the magnitude of the acceleration of the box at this instant.

I'm having trouble finding the acceleration in the normal direction so i can use
|a|=sqrt(at2+an2). I'm assuming av/dt=4m/s is the tangential acceleration.
maybe use an=v2/r but what is r?

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