What are you trying to solve for?
(You will end up with one of the variables on the RHS to be equal to some function of everything else ... which is it? Previously I just assumed you needed x(t).)
Check your original question and see if that is really intended to be a time derivative or not. Don't just go along with what we say.
What is "a" a function of?
You know how to differentiate a quotient right?