Given α21+β21=15
For equation 3x2+λx−1=0
α+β=3−λ, αβ=3−1⇒α2β2=91
then α2+β2=(α+β)2−2αβ=9λ2−2(3−1)=9λ2+32
⇒α2+β2=9λ2+6
Now α21+β21=15⇒α2β2α2+β2=15
⇒9λ2+6×α2β21=15 ⇒9λ2+6×911=15
⇒λ2+6=15 ⇒λ=±3
Now 6(α3+β3)2 =6((α+β)(α2−αβ+β2))2
=6×1×(915+31)2
=6×1×(2)2=6×4=24