not exactly sure what the question is...
couple of points though, it is usually easier to use a different dummy variable when you use a subtitution
say you have
[tex]\int_b^a dt f(t)[/tex]
and want to change to u(t) = t/2, then
t=2u
dt = 2.du
u(a) = a/2
u(b) = b/2
so the integral becomes
[tex]\int_{b/2}^{a/2} 2.du f(2u)[/tex]
see how this compares with your case...