Change base and divide me

Convert both numbers to their algebraic form in any base:

$221=2a^2+2a+1$

$1215=a^3+2a^2+a+5$

Then, do long division

The remainder must be $0$ in order to $221$ divides $1215 \Rightarrow -\frac{1}{2}a+4.5=0 \therefore \boxed{a=9}$

In base $9$ , $221$ divides $1215$