First, round both numbers to the nearest hundred.

6,391	→	6,400
605	→	600

Then, subtract.

The actual difference of 6,391 and 605 is 5,786.
So, 5,800 is a good estimate of the difference.

In a division problem, numbers are compatible if the dividend is easily divisible by the divisor.
To find compatible numbers, increase or decrease the dividend by a small amount so that it is divisible by the divisor.

Estimate 64 as 63, which is divisible by 7.