Given data as below:
a=2i^−7j^+5k^
b=i^+k^
c=i^+2j^−3k^
Given, r×a=c×a
⇒(r−c)×a=0
It means (r−c) is parallel to a,
∴r=c+λa.......(1)
Given, r⋅b=0
From equation (1),
(c+λa)⋅b=0
⇒(c⋅b)+λ(b⋅a)=0
⇒−2+λ(7)=0⇒λ=72
On putting values in equation (1) we get,
∴r=c+72a=71(11i^−11k^)
∣r∣=7112