Find the missing term.
the sum of 2ab2 and (-5ab2) is the same as the sum of (-6ab2)

2ab^2 + (-5ab^2) = 2ab^2 - 5ab^2 = - 3ab^2

the ab^2's stay the same...ur just messing with the coefficients.
so -3 + what is -6 ? -3 + (-3) = -6

therefore, -6ab^2 + (-3ab^2) = -6ab^2 - 3ab^2 = -3ab^2

so ur answer is : -3ab^2