Let three vectors a→,b→ and c→ be such that c→ is coplanar with a→ and b→,a→⋅c→=7 and b→ is  perpendicular to c→, where a→=−i^+j^+k^ and b→=−2i^+k^, then the value of 2|a→+b→+c→|2 is

c→=λ(b→×(a→×b→))=λ((b→⋅b→)a→−(b→⋅a→)b→)=λ(5(−i^+j^+k^)+2i^+k^)=λ(−3i^+5j^+6k^)c→a→=7⇒3λ+5λ+6λ=7⇒λ=12∴2a→+b→+c→2=2−32−1+2i^+52+1j^+(3+1+1)k^                                 =214+494+25                                =25+50=75