Question

Mgh=1/2mv^2solve for v

Answer 1

                                                           M g h  =  1/2 m v²

What you have here is the first step in solving a couple of
very interesting questions.

Question #1). 
I just lifted this thing and put it on the high shelf, so now it has
some potential energy.
How fast would it have to be moving to have the same amount
of kinetic energy ?

Question #2).
This thing is moving down the street, so it has some kinetic energy. 
How high would it have to be to have the same amount of potential
energy ?

The first step in solving either of those questions would be to SAY that
the potential energy is equal to the kinetic energy:

                                                M g h  =  1/2 m v²
                                                 (PE)   =    (KE)

The first thing I want to do to this equation
is to divide each side by ' m ' :         
                                                     g h  =  1/2 v²
There it is !
- If you know the height of the shelf (h) you can find the
speed (v) to give the equivalent KE.
- If you know the speed, you can find the height for the
equivalent PE.
-- And the mass of the object doesn't matter !

This question that you posted says: 
I know the height of the shelf.  It's ' h ' .  I know how much potential energy
the object on that shelf has.  How fast would it need to move to have the
same amount of kinetic energy ?

Solve this equation for ' v ' :     g h  =  1/2 v²

Multiply each side by  2 :       2 g h  =         v²

Square root each side:             v  =  √(2 g h)     
________________________________________

If the object were moving down the street with speed = v, you would
know its kinetic energy.  How high would you have to lift it so that it
had the same amount of potential energy ?      

Solve this equation for ' h ' :     g h  =  1/2 v²

Divide each side by ' g ' :            h  =  v² / 2g

Is that cool or what !

Answer 2

So since m is on both sides of the equal sign, they cancel each other out. Then multiply both sides by 2, so you get 2gh = v². Then (I'm assuming) g = acceleration due to gravity = 9.80 m/s². If you multiply it by the two, you get 19.6 m/s². I'm assuming h is the height, in meters. If you have the number for h, plug it in now and multiply that. You'll get some number x that has a value of m²/s². Since you're looking for v, and right now you only have v², take the square root of both sides of your equation and that'll give you the answer. If you don't have a value for h, though, just write your answer like √(2gh) = v or whatever format like that your teacher usually wants, and you should be good. I don't know how far the teacher wants you to simplify or solve this, but hopefully this gives you what you need! :)