However, this does not work.
My problem is not to plot U as a function of time, but somehow convert the function.
I mean, in Your solution U is defined through X(t) and thus Maple is evaluating the solution for DE every time the U is called.
However I would like to have U as function of t without precense of X(t). Since X(t) is a known procedure, there might be the way to explicitly define U as a function of t only, after the evlauation of procedure X(t).