Ok, so this is a projectiles question so you can split it into horizontal and vertical components. Let's make a checklist (look at this post on your computer or it won't look right)
Horizontal: Vertical:
velocity: ? inital velocity: 0m/s (assuming it is thrown from the tower exactly horizontally)
time: ? final velocity: ?
displacement: 15m time: ? (but will be same as horizontal time so we can call x)
displacement: 30m
acceleration: assuming 10m/s^2
so we can plug into equations. let's start with vertical:
s=ut+(1/2)at^2
30= 0x + (1/2)10x^2
...
x= Square root of 6
cool, so now you can work out horizontal velocity:
v=s/t
v=15/sqrt(6)
v= 6.12m/s
To be honest, I am not a physics genius, this is how I would do the question. may need someone to clarify if this is the correct way to do it. also, the question ain't that clear but assuming it is asking for horizontal velocity, this is the way to do it.
PRO PHYSICS TIP: always start with a list of what you know