How do i estimate 542,817
-27,398

Step-by-step explanation:

Much depends on how accurate an estimate you want.

If "to the nearest hundred" would be acceptable (it's a lot of work),

Round off 542817 to 542820 and 27398 to 2740.  Then you'd want to subtract 2740 from 542820.  (Again, it's a lot of work.)

Otherwise round off to the nearest thousand:  543000 - 27000.

Or go to the nearest ten thousand:  540,000 - 30,000 = 510,000 (estimate)