Is he square root of -144 rational or irrational?

Step-by-step explanation:

Neither.

It is a complex number. The - sign is the culprit. sqrt(-144) = i * sqrt(144) = 12 i

The "i" is an imaginary representation for the square root of -1. It is a definition for sqrt(-1). So a new category of number has been created. The ancients would have been profoundly surprised to learn that such a thing existed.