We know that by triangle law of vector addition,
a+b+c=0
⇒b+c=−a
Squaring both side we get,
∣b∣2+∣c∣2+2b⋅c=∣a∣2
Now putting the given value |\vec{a}|=6\sqrt{2},|\vec{b}|=2\sqrt{3}&\vec{b}.\vec{c}=12
We get, ∣c∣2=36
So, ∣c∣=6
Now, S1:∣a×b+c×b∣−∣c∣
⇒∣(a+c)×b∣−∣c∣
⇒0−6=−6
And S2:a+b+c=0
a+b=−c
Squaring both side we get,
∣a∣2+∣b∣2−2∣a∣∣b∣cos(∠ACB)=∣c∣2
⇒72+12−246cos(∠ACB)=36
⇒cos(∠ACB)=24648=62=32