hmm, interesting
one way would be to multply it out or set it equal to 11x where x is a whole number (if x is not a whole number, then it is not divisible)
11x=7^6+7^5-7^4
undistribute 7^4
11x=(7^4)(7^2+7^1-1)
11x=(7^4)(49+7-1)
11x=(7^4)(55)
56=5*11
11x=(7^4)(5)(11)
divide by 11
x=5(7^4)
aka, find if 11 is a factor of that number
x=5(7^4)