Find partial sums of the series $12+105+1008+10011+\dots$ 
Find the sum of $n$ terms of this series-
  $$12+105+1008+10011+.....$$

I did not understand that how should I proceed with this problem.

The series is:
$$\sum_{k=1}^n 10^k + 2 + 3(k-1) = \overbrace{\sum_{k=1}^n 10^k}^{S_1} + \overbrace{\sum_{k=1}^n 2}^{S_2} + \overbrace{\sum_{k=1}^n 3(k-1)}^{S_3} = \\ = \underbrace{\frac{10^{n+1}-1}{9}}_{S_1} + \underbrace{2n}_{S_2} + \underbrace{\frac{3n(n-1)}{2}}_{S_3} = \frac{10^{n+1}-1}{9} + \frac{n(3n+1)}{2}$$
