what is an equation I should use to find the fastest speed of a rollercoaster

If you're ignoring friction (i.e. if this is a homework problem), then you could calculate the change in gravitational potential energy from the point where the roller coaster begins, to the lowest point on the track. When the roller coaster is at the lowest point, all of that potential energy will have been converted into kinetic energy. Once you know the kinetic energy, you can easily calculate the velocity.

Note that you don't actually need to know the mass of the roller coaster to solve the problem, even though the formulas for the gravitational potential and kinetic energies both contain a mass term. That term will cancel from both sides once you set them equal to each other.

Can you take it from there, or do you need more details on how to do the calculations?