Assuming your numbers are rounded to the nearest km, the minimum area will be ...
(5.5 km)·(10.5 km) = 57.75 km² . . . minimum
And the maximum area will be ...
(6.5 km)·(11.5 km) = 74.75 km² . . . maximum
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When a number is rounded to 6 km as the nearest km, its value may actually be anywhere in the range 6 km ± 0.5 km. If you really want to get technical about it, the ranges of possible dimensions are [5.5, 6.5) km and [10.5, 11.5) km, and the range of possible areas is [57.75, 74.75) km².