We know that→a×(→b×→c)=(→a⋅→c)(→b)−(→a⋅→b)(→c)=5(^i+2^j+2^k)−6(^i+^j+2^k)=−^i+4^j−2^k
⇒→a×(→b×→c)=(1+α)^i+β(1+α)^j+γ(1+α)(1+β)^k=−^i+4^j−2^k
By comparing, we get
⇒1+α=−1,β(1+α)=4,γ(1+α)(1+β)=−2
α=−2,β(−1)=4⇒β=−4⇒γ(−1)(1−4)=−2
⇒γ(−1)(−3)=−2⇒γ=−23
⇒α−β−3γ=−2+4+2=4