3: We should start by noting that no (real) value of a, b, c will cause trouble in the limit, so the question makes sense for all values of parameters.

After substituting in the limit point you should get

If a is not zero, then x² is the dominant power in the numerator and pulls the whole ratio to (positive or negative) infinity. We confirm it for instance by factoring.
When a is zero, we are comparing two linear factors, this can be done for instance by cancelling.

Next hint
Answer