7^
0=1
1=7
2=49
3=343
4=2401
5=16807
you do not need to even write any other digit except the units digit.
its like this..
abjhbajsa7
x7
_________
tytiyqoiywt9
the units digit on one stage would produce the units digit of the next stage. the result would be independant of any other digit.
now.. as you saw.. the pattern is a set of 4's. every power which is of the form 4n+1 gives 7..4n+2 gives 9.. 4n+3 gives 3 and 4n+0 = 1
2007 mod 4 = 3
2004+3 = 4n + 3
so the units digit is 3
i hope i am right