Convert base-6 to base-21

$N=n_i=\sum _{i=0}^n{d}_i*6^i$ $N\text{'}=\sum _{j=0}^{\infty }{d}_j*{21}^{-j}$ Note: In these formulas, "N" is the original number in base-6, "n" is the number of digits, "di" is the i-th digit from the right, and "N'" is the resulting number in base-21.