For a reaction with the rate equation rate = k[A][B]^2, the overall order of the reaction is

Explanation

Overall order is the sum of the individual orders, so 1 for A plus 2 for B gives 3. The reaction is first order in A and second order in B, and doubling the concentration of B alone would therefore quadruple the rate while doubling A alone would only double it. Adding rather than multiplying the exponents is the step candidates most often get wrong.