Step 1: Find the direction ratios of the line.
The given points are
\[
A(4,3,-5)
\]
and
\[
B(-2,1,-8)
\]
Direction ratios are obtained by subtracting corresponding coordinates:
\[
a=-2-4=-6
\]
\[
b=1-3=-2
\]
\[
c=-8-(-5)=-3
\]
Thus, one set of direction ratios is
\[
(a,b,c)=(-6,-2,-3)
\]
Step 2: Find the coordinates of point \(P\).
Given,
\[
P(a,3b,2c)
\]
Substitute the values of \(a,b,c\):
\[
P=(-6,3(-2),2(-3))
\]
\[
P=(-6,-6,-6)
\]
Step 3: Check which plane satisfies the point.
For plane
\[
x+y-2z=0,
\]
substitute \((-6,-6,-6)\):
\[
-6+(-6)-2(-6)
\]
\[
=-12+12
\]
\[
=0
\]
Hence, the point satisfies the plane equation.
Step 4: Final conclusion.
Therefore, the point \(P(a,3b,2c)\) lies on the plane
\[
\boxed{x+y-2z=0}
\]