1. The ratio of horizontal distance between red curve and blue curve is 2. Because dx= vx*dt, [color=red]vx*dt[/color] = 2 [color=blue]vx*dt[/color]

2. The velocity of vy will be zero when it reach the top point. Because vy=vy[sub]0[/sub] -gt

So the time to reach the top (=dt/2) will doubled if the initial velocity is also doubled.

The maximum height h is proportional to t[sup]2[/sup]: if you double velocity in vertical direction, the time to reach the top will be doubled, and the distance is velocity * time, so distance will be 4 times (2 dt * 2 vy=4*vy*dt)

Since [color=red]dt[/color] = [color=blue]dt [/color]

So the only possible solution is Vx=?2 Vx, Vy= ?2 Vy