Why does $(6+i)^3 = 198+107i$?

Question

When I expand it I get $i^3+18i^2+108i+216$. How does one go from that to $198+107i$. I noticed that the 1st term of the 2nd equation is = to the 4th term of the 1st equation - the 2nd term of the 1st equation, and the 2nd term of the 2nd equation is = to the 3rd term of the 1st equation - the 1st term of the 1st equation. Is this relevant or coincidental? Help is greatly appreciated, thank you!

Answer 1

Note that $i^2=-1$ and so $i^3 = -i$ from here you can easily see why it’s true .

Answer 2

Since $i^2 = -1$, you can substitute $18i^2$ with $-18$.
$216 - 18$ gives us 198. Since $i^3 = -1 \cdot i = -i$, we can substitute $i^3$ with $-i$. $108i - (1)i = 107i$.