Concept:
The required line passes through all common points of the two parabolas. Hence, first find their points of intersection and then impose the condition that both points satisfy the line.
Step 1: Find the points of intersection of the parabolas.
Given
\[
y^2=4ax
\]
and
\[
x^2=4ay.
\]
From the first equation,
\[
x=\frac{y^2}{4a}.
\]
Substituting into the second equation,
\[
\left(\frac{y^2}{4a}\right)^2
=
4ay.
\]
\[
\frac{y^4}{16a^2}
=
4ay.
\]
\[
y^4
=
64a^3y.
\]
\[
y(y^3-64a^3)=0.
\]
Therefore,
\[
y=0
\]
or
\[
y=4a.
\]
Hence the corresponding points are
\[
(0,0)
\]
and
\[
(4a,4a).
\]
Step 2: Use the fact that the line passes through \((0,0)\).
Substituting \((0,0)\) into
\[
2bx+3cy+4d=0,
\]
we get
\[
4d=0.
\]
\[
d=0.
\]
Step 3: Use the point \((4a,4a)\).
Substituting \((4a,4a)\),
\[
2b(4a)+3c(4a)+4d=0.
\]
Since \(d=0\),
\[
8ab+12ac=0.
\]
\[
4a(2b+3c)=0.
\]
Given
\[
a\neq 0,
\]
therefore,
\[
2b+3c=0.
\]
Step 4: Combine the two conditions.
We have
\[
d=0
\]
and
\[
2b+3c=0.
\]
Hence,
\[
d^2+(2b+3c)^2=0.
\]
Step 5: Write the final answer.
\[
\boxed{d^2+(2b+3c)^2=0}
\]