=4⇒r=7 ∴S(p,q,r)=S(5,6,7) Now, equation of line CS will be
x−5
5−3
=
y−6
6−5
=
z−7
7−9
x−5
−2
=
y−6
−1
=
z−7
2
=λ (say) ∴x=−2λ+5,y=6−λ,z=2λ+7 Thus, coordinate of B will be ((−2λ+5),(6−λ),(2λ+7)) Since, B lies on given plane.∴(−2λ+5)+(6−λ)+(2λ+7)=12 ⇒λ=6 ∴B=(−7,0,9) ∴OB=√(−7−0)2+(0−0)2+(19−0)2=√410