3a+λb+7c=0
故3a+λb=-7c,两边取模|3a+λb|=|-7c|=7
两边平方9a^2+6λa*b+λ^2b^2=49
又a^2=b^2=1,a*b=1/2
故λ^2+9+λ*3=49
λ=-8或5