Let a→=i^-2j^+k^ and b→=i^-j^+k^ be two vectors. If c→ is a vector such that b→×c→=b→×a→ and c→·a→=0, then c→·b→ is equal to:

given that b→×c→=b→×a→cross product with a→ on both sidesa→×(b¯×c¯)=a¯×(b¯×a¯) ⇒(a→·c⇀)b⇀-(a→·b⇀)c⇀=a→.a→b⇀-a⇀.b⇀a⇀0⇀-4c→=6(i^-j^+k^)-4(i^-2j^+k^)      -4c⇀=2i⇀+j⇀+k⇀           c⇀.b⇀=-12i→+j→+k→.i→-j→+k→ =-12