Applications for Derivatives (velocity)

*insert long descriptive paragraph about a volcano*

Paraphrase: With regards to a shot of lava at a point 1900 ft shooting straight into the air, what was the lava's exit velocity in feet per second? in miles per hour?

[HInt: If v0 is the exit velocity of a particle of lava, its height t seconds later will be s= v0t - 16t^2 feet. Begin by finding the time at which ds/dt= 0. Neglect air resistance]

I assume v0 is just another term for the first velocity.

ds/dt= v0 - 32t

v0 - 32t = 0

v0 = 32t

That seems unhelpful.

What now? or did I misunderstand?

Thanks!