Let A be (3λ+7,−λ+1,λ−2) and B be (2μ,3μ+7,μ)
DR's of AB is
(3λ−2μ+7,−λ−3μ−6,λ−μ−2)
Now,
13λ−2μ+7=−4−λ−3μ−6=2λ−μ−2
i.e. −12λ+8μ−28=−λ−3μ−6
⇒λ=μ−2
Also
−2λ−6μ−12=−4λ+4μ+8
⇒λ=5μ+10
After solving above two equations, we get λ=−5,μ=−3
i.e. A=(−8,6,−7) and B=(−6,−2,−3)
Hence, (AB)2=4+64+16=84