ans
shows up because you call
LaplaceTransform(bla, blabla)
instead of
LaplaceTransform(bla, blabla);
(you lack a semicolon when you call the function).
exact_answer
shows up because your line
exact_answer=antiderivative(b,s)-antiderivative(a,s)
lacks a semicolon as well, you should have
exact_answer=antiderivative(b,s)-antiderivative(a,s);