The key to this question is that you know after a time, T the vertical displacement is zero. So working the vertical plane you need to find at which times, T_1 and T_2 the projectile has a vertical displacement of zero, one of the values will be zero obviously. After which you can use your non zero value (or expression..) of time into your horizontal component in order to work out v. I don't want to just give you the answer, but have a fiddle with these The equations of constant acceleration are: s=ut+0.5at^2 v=u+at v^2 = u^2 +2as s=((u+v)/2) t