th image of point p(1,3,4) in th plane 2x-y+z+3 is?

Given a point P(1,3,4)

Let Q be the image of the point.

the dr''s of PQ is (2,-1,1)

Eqn of line PQ= x-1/2 = y-3/-1 = z-4/1

Any point on line PQ is (2r+1, -r+3, r+4)

If this point is on the plane , then

2(2r+1)-(-r+3)+(r+4)+3=0

4r+2+r-3+r+4+3=0

6r+6=0

r=-1

Therefore coordinates of foot of the perpendicular (say S) = (-1,4,2)

We know that S is the middle point of PQ 

=> -1=x1+1/2, 4=y1+3/2, 3=4+z1/2

=> x1=-2, y1=5, z1=2

Therefore , Image of the point P is Q(-2,5,2).