From the given data rate law for the reaction
Rate = K[a][b]2
Rate of reaction when concentration of all reactants are set at unity , R=K
therefore rate constant = 1x 10-2
From rate law rate of reaction when conc becomes half is R=K[0.5][0.5]2 =1x 10-2[0.5][0.5]2 =1.25 x 10-3molsL-1s-1