Strictly speaking, what you want to prove here is NOT true! If and , the first two conditions are incompatible. I am going to assume that .

Since that third condition follows from the first so we really have only two conditions. That means we are free to choose one of the parameters to be whatever we want. Choosing makes the first condition . With and , the second condition becomes which gives as long as .

If we have and , then we really only have one condition. Choose and . a now can be any number at all.