L = √(1 + 4 x6) /(2 - x3) can be rearranged by pulling a factor of x3 out of the numerator and denominator.
These factors of x3 cancel leaving
L = √(4 + 1/x6 ) / (2/x3 -1)
In this form, the limit as x →∞ is straightforward because the limit x →∞ of both 1/x3 and 1/x6 = 0
So lim x→∞ L = √4/(-1) = -2