Given,
a,b and c be three non-zero non-coplanar vectors,
Also the position vectors of four points A,B,C and D be a−b+c,λa−3b+4c, −a+2b−3c and 2a−4b+6c respectively,
So, AB=(λ−1)a−2b+3c
AC=−2a+3b−4c
AD=a−3b+5c
Now using the condition of coplanar we get,
∣λ−1−21−23−33−45∣=0
⇒(λ−1)(15−12)+2(−10+4)+3(6−3)=0
⇒(λ−1)=1⇒λ=2