Here $-3$ to $3$ are the digits, with corresponding values $-3 = C, -2=B, -1=A, 0, 1, 2$ and $3.$ Then we will have to find values which satisfy the equation:
$d_1 + 7{d_2}+49d_3+343d_4+…. = 102$, where $d_1,d_2…$ are from $\{-3,-2,-1,0,1,2,3\}$. Then we will get $d_1 = -3,
d_2 = 1, d_3=2 $ and $ d_4 =0.$
That is $21(-3)$, which is $21C$ in the given system.