Answer
$12.054\space h$
Work Step by Step
We can write,
New rotation period = previous rotation period - decreased time
$\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space =\space 12.404\space h\space -\space 21\space min$
Now we have to convert minutes into hours. We know 1 h = 60 min, so we can use the conversion factor 1 h / 60 min and give the value as follows.
$21\space min\space=\space 21\space min\space \times\space(\frac{1\space h}{60\space min})=0.35\space h$
Therefore,
$New\space rotation\space period = 12.404\space h\space -\space 0.35\space h $
$\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space\space \space \space \space \space \space = 12.054\space h$
