How do i find X if i need X to find the way to calculate x? (trajectory)

[strong lintel]

I'm wanting to find how far it can go. This does not make sense to me as I need X To get the hypotenuse to work out what I need to work out the Hypotonuse. Can someone explain how I would do this?
(Balistic trajectory using a set velocity)

[strong lintel]

I then have gone onto the persons actual code (different language so I can't just steal it waa waa) and they explained it more in the notes

But how do I get X if I'm trying to find X and the only way I can get X is if I have X to get the hypotonuse to find X!?