As the given vectors are coplanar, so ∣23c113c231c3∣=0
⇒8c1−7c2−3c3=0...(i)
Also, a⋅c=5⇒2c1+c2+3c3=5...(ii)
and b⋅c=0⇒3c1+3c2+c3=0...(iii)
Solving the above three equations using Cramer's rule, we get,
Δ=∣823−713−331∣=−122
Δ1=∣050−713−331∣=−10
Δ2=∣823050−331∣=85
Δ3=∣823−713050∣=−225
Hence, 122(c1+c2+c3)=122(12210−12285+122225)=150