ok...i am pretty sure i can help you witht his one :)
if you have been learning projectile motion in class then you should know:
[tex]\Delta Y = (V_{0}sin\varphi)(t)-.5gt^{2}[/tex]
where [tex]\varphi = 45 degrees (the angle of "launch"[/tex]
also you should know:
[tex]\Delta X = (V_{0}cos\varphi)(t)[/tex]

NOW here is what you will have to do:
solve the change in X function for t and plug that in for wherever "t" appears in the change in Y equation
set the change in Y to equal 1.7 m (the ball needs to at least hit that point to make it over the wall) and then solve that big nasty equation algebraically to get [tex]\ (V_{0} [/tex]

***edit : sorry its my first time with the math code so ill write this out
phi = 45 cuz its the angle of launch (the writing was kinda hard to read)
hope i was helpful!