$$given: T(n)=T(n-1)+n^3 ; T(1)=1\\=T((n-1)-1)+(n-1)^3+n^3\\=T(n-2)+(n-1)^3+n^3\\=T((n-1)-2)+(n-1-1)^3+(n-1)^3+n^3\\=T(n-3)+(n-2)^3+(n-1)^3+n^3\\…\\=T(n-k)+(n-k-1)^3+(n-k-2)^3+⋯+n^3\\n-k=1\\k=n-1\\=T(1)+(n-1)^3+(n-2)^3+⋯+n^3$$
I'm stuck here, does this look right/what would then next step be?