What's the fastest way?

If you took the time of comparing this equation with $y=x\tan\theta-\dfrac{gx^2}{2u^2\cos^2\theta}$ or $y=x\tan\theta\left(1-\dfrac{x}{R}\right)$ any other slower method, here's the solution.

At the end point, i.e., the point where the projectile meets the x-axis for the second time, the range is covered and the y-coordinate is zero. Hence substituting $y=0$ , we get $x=9$ .