you just multiply the sides.
Firstly, it's easier if you convert both measurements to inches.
so you'd do 6 x 12 = 72 , 72 + 5 = 77 inches
and 4 x 12 = 48 , 48 + 3 = 51 inches.
Then you multiply the sides, so:
77 x 51 = 3927 inches
then you divide by 12 to get how many feet and inches it is
3927/12 = 327 feet and 4 inches