That would be the quadratic formula. If you can’t see how to apply it and this isn’t some kind of homework problem, post again and I’m sure we can provide more details.

If you’re asking this out of sincere befuddlement, it’s because a single equation with terms involving three variables can’t be expressed without all three variables. So if you isolate one on one side of the equation, you must have the other two on the other side. (Okay, technically you can make a trivial example by having terms that cancel out, but that’s not the case here).
Since the title of the OP had N=, that would seem to be what’s desired. It’s possible that in their case, t is considered a known constant rather than a variable. Otherwise it is incorrect to say that this “in terms of X” instead of “in terms of X and t” (which, if you weren’t confused about the basic concept, was probably your point).

I think we are predisposed to think that t means time and is a variable, but it doesn’t have to be so. If the letter were a instead of t, it might look more like a constant and solving for X might seem more familiar.

Yeah, sorry for the confusion, t is a constant. I should have invert the casing of the alphabets (guess this shows long I have left school) as constants are typically capital letters.

I am still working out the formula. Trivial: The equation is how much experience points you need to reach the Nth level in Dungeons and Dragons.