Concept:
Direction ratios of the line must satisfy plane equation.
Step 1: Direction ratios of line.
\[
(l, m, n)
\]
Step 2: Plane contains line.
Thus every point on line satisfies:
\[
a(x-\alpha)+b(y-\beta)+c(z-\gamma)=0
\]
Step 3: Substitute parametric form.
\[
x=\alpha+lt, y=\beta+mt, z=\gamma+nt
\]
Substitute:
\[
a(lt)+b(mt)+c(nt)=0
\]
\[
t(al+bm+cn)=0
\]
Step 4: Conclusion.
Since true for all \(t\):
\[
al+bm+cn=0
\]