I'll add onto the hint. Newton first law, in a nutshell, says that if the net force is equal to zero then the body's velocity is not changing, so since the velocity isn't changing then accelaration isn't there.
So draw a free body diagram. What forces are acting upon the cart? Do we...
Let us look at what we know.
We know acceleration is 9.8 m/s because once an object is released then free fall takes place. We also know the height of the object at a certain distance. So, can you find a formula that uses those three variables plus one missing one?
You were correct in saying that the distance would be defined as 2D.
So now we must define time. We know that 2D must be divided by time, but how do we define time? Simple, were change the formula to define it as
2D all over D/v going up + D/v going down.
Do some cancellations and then...