6^big

here it is:

$6^1=6$

$6^2=36$

$6^3=216$

$6^4=1296$

$6^5=7776$

$..........................$

$..................$

every time, the last digit is 6.so, the last digit will be 6.

${\color{#D61F06}{6}}^2=3{\color{#D61F06}{6}}$ So it is clear that for any $n>0$ integer, $6^n$ ends in a $\boxed{6}$ .

The last digit of $6^{123655653648596098976557878765643654675869789}$ equals to $6^{1}$ , so it is $\boxed{6}$ .

$6$ to the power any number will always have 6 in the units digit.