Determine the following limit.

lim (1/x-3)
x->3

I assume you are missing parentheses, namely you mean:

Lim 1/(x-3)
x->3

When the numerator evaluates to a finite number, and the denominator to zero, then the limit would be infinite.
L'Hôpital's rule applies only when both numerator and denominator evaluates to infinity or both zero.