The d.r.s. of the normal to the plane are 1,−1,2. ∴ The equation of line PM is
x−2
1
=
y−4
−1
=
z+1
2
=λ (say) ⇒x=λ+2,y=−λ+4,z=2λ−1 Let M≡(λ+2,−λ+4,2λ−1) ∴ Equation of plane becomes 1(λ+2)−1(−λ+4)+2(2λ−1)−2=0 ⇒λ=1 ∴M≡(3,3,1) Since M is the mid-point of PQ. ∴