Given:
A(2,6,2),B(−4,0,λ),C(2,3,−1)D(4,5,0)
Area of quadrilateral
=21∣AC×BD∣=18
⇒∣AC×BD∣36
Now,
AC×BD=∣i^08j^−35k^−3−λ∣
⇒AC×BD=(3λ+15)i^−(0+24)j^+(0+24)k^
⇒AC×BD=(3λ+15)i^−24j^+24k^
So,
∣AC×BD∣=36
⇒(3λ+15)2+(24)2+(24)2=36
⇒9λ2+225+90λ=1296−1152
⇒9λ2+90λ−81=0
=λ2+10λ+9=0
⇒λ=−1,−9
Since,
∣λ∣≤5⇒λ=−1
Therefore,
5−6λ=5−6(−1)=11