6. What is the tens' digit of the largest 4-digit even number which uses each of the digits 5, 7, 8, and 9 exactly once?
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The subtraction theorem states that for all real numbers, 
and 
, 
.
(To subtract, we can add the inverse.)
Thus, we can have the these two equivalent expressions.
(To subtract, we can add the inverse.)
Thus, we can have the these two equivalent expressions.