Answer
$-2ab^2$
Work Step by Step
To combine like terms, you add/subtract the coefficients. When a variable term has no coefficient, the implied coefficient is 1.
$-ab^2-ab^2\longrightarrow$ write the coefficient
=$-1ab^2-1ab^2\longrightarrow$ use the distributive property to factor out the coefficients
=$(-1-1)(ab^2)\longrightarrow$ subtract
=$-2ab^2$
