Formula : -

Given: -

we know that vector are coplanar if their scalar triple product is zero

we have

now, using

⇒ 0 = 1(λ – 1) + 1(2λ + λ) + 1( – 2 – λ)

⇒ 0 = λ – 1 + 3 λ – 2 – λ

⇒ 0 = 3 λ – 3

⇒ λ = 1