The criteria is
Between 60 & 100
Units digit 2 less than tens
Prime number
79 is a number between 60 & 100 and it is a prime number (as its only factors are 1 and itself), however 79 has a tens digit that is 2 less than the units - not the units 2 less than the tens.
As it does not comply with one of the rules the number cannot be 79
The number that would work is 97
