I hope user do not need to use the above equations to solve the problem. Instead, ?hope user can use physics reasoning to solve it. Click the image to find out what I mean:

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