Explanation:
if the one-sided limits (that means approaching the target from the right only and from the left only) are different numbers, then the general (two-sided) limit does not exist.
that is the case here.
lim c- f(x) at 1 = 1 - 1 = 0
lim c+ f(x) at 1 = 2×1 = 2
and f(1) = 3
the general limit does not exist at this point.