A certain reaction is zero order in reactant a and second order in reactant

b. if concentration of both reactants are doubled, what happens to the reaction rate?

As we know that, for zero order reaction the rate of reaction is constant and does not change with the concentration of reactant. So, doubling concentration of reactant a will have no affect on the rate of reaction. However, for second order the reaction rate is proportional to the square of concentration of reactant. Hence, if concentration of b is doubled, ration rate will be quadrupled. So, overall reaction rate will get four times its initial rate, if concentrations of both reactants are doubled.