Formula : -

and then,

(iii) Three vectors are coplanar if and only if a⃗.(b⃗×c⃗) = 0

Given: -

we know that three vector a⃗,b⃗,c⃗ are coplanar if their scalar triple product is zero

we have

now, using

= 1(15 – 12) + 2( – 10 + 4) + 3(6 – 3)

= 3 – 12 + 9 = 0