Question: expand as multipolynomial

I have this expression:


I have to rewrite this as a multipolynomial in the variables x and sin(y).

I'm trying this:

expand((a*x^2+b*x*sin(y)+c*sin(y))^2+(a*sin(x)^2+b*x)^3, x,sin(y));

but i get the same result as expand without the 2 variables at the end.

am i doing it right?

Thanks in advance!

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