One of the equations of gravity is this:
[tex] {v}^{2} = {u}^{2} + 2gh[/tex]
Where v = final velocity which is 7m/s
u = initial velocity which is 0 for objects falling from a height
g = acceleration due to gravity and it is approximately 10m/s^2. It's a constant so pretty much remember this number. It's positive since the work being done is caused by gravity (in other words, it's falling down). It can also be negative if the work being down is against gravity (in other words, it's going up)
h = height of object
Substitute for the values and you should have something like this
[tex] {7}^{2} = {0}^{2} + 2 \times 10 \times h[/tex]
[tex]49 = 0 + 20h[/tex]
[tex]h = \frac{49}{20} [/tex]
[tex]h = 2.5m[/tex]
