Finding final velocity

1. The problem statement, all variables and given/known data
A marble is released from a 10cm ramp. Its final velocity is 1.4 m/s. If it is released from a 20cm ramp, twice the height, what will its final velocity be?

2. Relevant equations
E = mgy + mv^2/2
mgy = mv^2/2 (Like initial gravitational potential energy will equal final kinetic energy? Does that make sense?)

3. The attempt at a solution
I tried to use mgy = mv^2/2, where I get 2gy = v^2. For g, our physics teacher has us use 10 m/s^2, and if the height is 20cm, then (2*.2m*10m/s^2) = v^2, 400 = v^2, v = 2m/s, but I don’t think that is right.

I asked my teacher for help, and he said to think about the relationship between the height and the velocity (which I think has to do with square roots, but other than that, I am lost).

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