Let a=αi^+j^+βk^,b=3i^−5j^+4k^
Now finding a×b=−i^+9j^+12k^
=∣i^α3j^1−5k^β4∣
⇒(4+5β)i^+(3β−4α)j^+(−5α−3)k^
=−i^+9j^+12k^
Now on comparing side we get,
4+5β=−1,3β−4α=9,−5α−3=12
β=−1,α=−3
So, \vec{a}=-3\hat{i}+\hat{j}-\hat{k}&\vec{b}=3\hat{i}-5\hat{j}+4\hat{k}
So,a+b=−4j^+3k^
Now finding {|\vec{a}|}^{2}=11&{|\vec{b}|}^{2}=50
And a⋅b=−9+(−5)−4=−18
Now projection of (b−2a) on a+b will be
=∣a+b∣(b−2a)⋅(a+b)
=∣a+b∣∣b∣2−2∣a∣2−(a⋅b)=550−22−(−18)=546