When I simplify $(3a-b^2-a)^3$, I used $(u±v)^3=u^3±3u^2v+3uv^2±v^3$
but I'm confused with $(b^2-a)$
When I simplify $(3a-b^2-a)^3$, I used $(u±v)^3=u^3±3u^2v+3uv^2±v^3$
but I'm confused with $(b^2-a)$
First of all, your cited identity is incorrect; the exponent on the $v$ was wrong in the last term. The correct version is $$(u\pm v)^3=u^3\pm 3u^2v + 3uv^2\pm v^3.$$ Also, note that $$3a-b^2-a=2a-b^2$$ so that $$(3a-b^2-a)^3=(2a-b^2)^3,$$ and you can then let $u=2a$ and $v=-b^2$, making the problem somewhat more straightforward.