Given,
a=λi^+μj^+4k^,b=−2i^+4j^−2k^ and c=2i^+3j^+k^ are coplanar
So, ∣λ−22μ434−21∣=0
⇒λ(10)−μ(2)+4(−14)=0
⇒10λ−2μ=56
⇒5λ−μ=28.....(1)
And given projection of a on b is 54
⇒∣b∣a⋅b=54
⇒24−2λ+4μ−8=54
⇒−2λ+4μ−8=54×24....(2)
By solving equation (1)&(2) we get,
⇒λ+μ=24