When you use "u-substitution" with a definite integral you have to find the new limits for the integral.
If the lower limit in the integral is $a$ and the higher limit $b$ (and o is for original and p for after u substitution) and
$u = f(x)$
$pa = f(oa)$ and $pb = (ob)$
Sometimes this results in $pb<pa$
e.g. $u = 10 - x $, $oa = -10$ and $ob = 6$ then $pa = 20$ and $pb = 4$
What happens in a situation like this? Do you swap the limits and make the limit with the higher value the higher limit?