Given:
a×c=b×c
⇒a×c−b×c=0
⇒(a−b)×c=0
So, (a−b)∥c, hence
a−b=μc
where, μ is any scalar.
⇒−2i^+7j^+2λk^=μc....(1)
Taking dot product with a, we get
⇒−2+14+2λ2=7μ
⇒7μ=2λ2+12...(2)
Taking dot product with b in (1), we get
-6-35-2\lambda 2=-432\mu
⇒82+4λ2=43μ....(3)
Solving (2)&(3), we get
λ2=1,μ=2
So,
a⋅b=3−10−λ2=−8
⇒∣a⋅b∣=8