Let $S=7n+70(n-1)+700(n-2)+\cdots+7\times10^{n-1}(1)+7\times10^{n}(0)$.
$10S=70n+700(n-1)+7000(n-2)+\cdots+7\times10^{n}(1)+7\times10^{n+1}(0)$
Subtracting the first equation from the second:
$9S=-7n+70[n-(n-1)]+700[(n-1)-(n-2)]+7000[(n-2)-(n-3)]+\cdots+7\times10^{n}(1-0)+7\times10^{n+1}(0)$
$9S=-7n+70+700+7000+\cdots+7\times10^{n}$
$9S=-7n+\dfrac{70(10^n-1)}{10-1}$
$S=-\dfrac79n+\dfrac{70(10^n-1)}{81}$
$S=-7\left(9n-\dfrac{10(10^n-1)}{81}\right)$
$S=-\dfrac7{81}(9n-10(10^n-1))$
$S=-\dfrac7{81}(9n-10^{n+1}+10)$