linear algebra – Constructing a basis that includes $1-t^3$ and $1+t^3$?

The question goes like: Construct a basis that includes $1-t^3$ and $1+t^3$ for the vector space of polynomials in variable $t$, where the $degree$ of polynomial is $<=3$.

So we have to construct a basis which already includes $1-t^3$ and $1+t^3$, that means we have to find just one more value. I tried some rough calculations that didn’t make much sense and got another vector as $t^3$. How should I go upon solving this problem?