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We only need to look at the last digit.
7 ^ 1998 ^ 1999

The pattern of the units digit of 7 
When exponent is divided by 4,
r1     r2     r3     r0
7      9       3      1

Take note that when we divide by 4 we only need to look at the last two digits.

When 1998 is divided by 4 we have a remainder of 2, 1999 we would have a remainder of 3.
Then the remainder of 1998*1999 when divided by 4 is (2*3)mod 4 = 6 mod 4 = 2 mod 4

Therefore the last digit would be 9.