Concept:
Two circles cutting orthogonally satisfy:
\[
2g_1g_2+2f_1f_2=c_1+c_2
\]
Maximum chord occurs when line passes through center.
Step 1: Equation of variable circle.
Passing through origin.
\[
x^2+y^2+2gx+2fy=0
\]
Step 2: Orthogonal condition.
Given second circle:
\[
g=-3,\qquad f=4,\qquad c=16
\]
Apply condition
\[
2(g)(-3)+2(f)(4)=16
\]
\[
-6g+8f=16
\]
\[
-3g+4f=8
\]
Step 3: Maximum chord condition.
Chord maximum when line passes through center.
Center of circle
\[
(-g,-f)
\]
Must lie on
\[
x-y-2=0
\]
So
\[
-g+f-2=0
\]
\[
f-g=2
\]
Solve equations.
\[
g=-1,\qquad f=1
\]
Diameter line through center and origin:
\[
x+y=2
\]
Thus
\[
\boxed{x+y=2}
\]