The paper 'Leibniz rule for fractional derivatives generalized and an application to infinite series' seems to be useful for you. Follow the link below to check out
I would say that the answer for your question is very complicated in the general case. If I understood correctly, the unknown x in the file is a function of t. Then you not only need the Leibniz formula, but also the chain rule for fractional derivatives. You can find these rules e.g. from Chapter 3 of the book "The Analysis of Fractional Differential Equations - An Application-Oriented Exposition Using Differential Operators of Caputo Type" by K. Diethelm.
Even in the simplest cases the answer cannot be expressed in terms of elementary functions. For example, if b=infinity and x(t)=t, you will end up to the Beta function provided 0