113 = 2 x 7² + 2 x 7¹ + 1 x 7⁰ = 221₇

(How can I get those number?

CLUE: Keep dividing 113 by 7:

113 \(\div\) 7 = 16 (remainder 1)

16 \(\div\) 7 = 2 (remainder 2)

Because 2  \(\div\) 7 = 0, we stop here.

We get three number: 2, 2, 1)