What is the tens digit of the largest four-digit even number which uses each of the digits 578 and 9 exactly once?

5.

The only even digit is 8, so that must be last. The remains 3 digits 5, 7, 9 form the largest number when the highest digit is in the highest place value, the next highest in the next highest place value and so on, ie 975, giving the four-digit even number 9758 with 5 in the tens place.