The rate of reaction of an irreversible second order bimolecular reaction of species

A and B can be written as:

where and represent the initial concentration of reactants A and B respectively, k is the reaction rate constant and is the conversion of the limiting reactant A.

Find an expression for the conversion of A in terms of time for the above reaction.

I thought of simply rearranging the formula to get it in terms of dxa/dt, since this is the change of the concentration of the reactant A with respect to time, however it is not the correct answer. Can someone please explain how to start this? I dont understand what I am being asked to do?

Thank you.