Consider a filter with real-valued impulse response $h[n]$. The filter is cascaded with another filter whose impulse response is $h'[n] = h[-n]$, i.e. whose impulse response is the time-reversed version of $h[n]$.
$h[n]$ cascaded with $h[-n]$
i think we have to perform convolution
if so what is phase of new cascaded impulse response