Given, a×(2i^+k^)=2i^−13j^−4k^
Now multiplying both side by (3i^+21j^+2k^) we get,
⇒(a×(2i^+k^))×(3i^+21j^+2k^)
=(2i^−13j^−4k^)×(3i^+21j^+2k^)
⇒((3i^+21j^+2k^).a)(2i^+k^)−(6+2)a=∣i^23j^−1321k^−42∣
⇒0−(6+2)a=∣i^23j^−1321k^−42∣
a=3i^+2j^−5k^
Now, projection of a on vector 2i^+2j^+k^ is a⋅3(2i^+2j^+k^)=35