(b)
We have,
$p^{'}(x) = p^{'}(1 - x),\forall x \in \lbrack 0,1\rbrack,p(0) = 1,p(1) = 41$
$\Rightarrow \ p(x) = - p(1 - x) + C$
At $x = 0,p(0) = - p(1) + C$
$\Longrightarrow \ \ 1 = - 41 + C$
$\Rightarrow \ C = 42$
$\therefore p(x) + p(1 - x) = 42$
Now, $I = \int_{0}^{1}{p(x)dx = \int_{0}^{1}{p(1 - x)dx}}$
$\Rightarrow \ 2I = \int_{0}^{1}{\left( p(x) + p(1 - x) \right)dx = \int_{4}^{1}42dx = 42}$
$\Rightarrow \ I = 21$