Given, ∣a∣=9
And (xa+yb)⋅(6ya−18xb)=0as they are perpendicular
⇒6xy∣a∣2−18x2(a⋅b)+6y2(a⋅b)−18xy∣b∣2=0
⇒6xy(∣a∣2−3∣b∣2)+(a⋅b)(y2−3x2)=0
This should hold ∀x,y∈R×R−(0,0)
So, {|\vec{a}|}^{2}-3{|\vec{b}|}^{2}=0&(\vec{a}\cdot \vec{b})=0
\therefore {|\vec{a}|}^{2}=3{|\vec{b}|}^{2}&(\vec{a}\cdot \vec{b})=0\Rightarrow \mathrm{cos}\theta =0\Rightarrow \theta =90^{\circ}
Now ∣a×b∣2=∣a∣2∣b∣2sin2θ=∣a∣2∣b∣2as θ=90∘
=∣a∣2⋅3∣a∣2 as ∣b∣2=3∣a∣2
So, ∣a×b∣=3∣a∣2=381=273