Explanation:
if there is no typo, then it is 4th degree for b.
let's rewrite here what I can see :
3a²ab - 5a²b²b² - 12a²b + 2a²b² - 18
that is simplified
3a³b - 5a²b⁴ - 12a²b + 2a²b² - 18
the highest exponent is 4 for b.
and so, this is the degree of the polynomial.