Answer :
[tex]\{u,v,w\}[/tex] forms a basis for [tex]V[/tex], which means any vector [tex]x\in V[/tex] can be written as the linear combination,
[tex]x=c_1u+c_2v+c_3w[/tex]
Pick [tex]c_1=d_1+d_2[/tex], [tex]c_2=d_1+d_3[/tex], and [tex]c_3=d_2+d_3[/tex]; then
[tex]x=(d_1+d_2)u+(d_1+d_3)v+(d_2+d_3)w[/tex]
[tex]x=d_1(u+v)+d_2(u+w)+d_3(v+w)[/tex]
Any [tex]x\in V[/tex] can thus be written as a linear combination of the vectors [tex]\{u+v,u+w,v+w\}[/tex], so these three vectors also form a basis for [tex]V[/tex].