Step 1: Concept
Under translation of axes, only the constant term changes after choosing the new origin to eliminate linear terms.
Step 2: Meaning
Put
\[
x=X+\frac74,\qquad
y=Y-\frac14.
\]
The transformed equation has no linear terms.
Step 3: Analysis
Substituting the new origin coordinates into
\[
x^2-2xy+3y^2+2gx+2fy-6,
\]
and equating coefficients of $X$ and $Y$ to zero gives
\[
g=-2,\qquad f=2.
\]
The resulting constant term becomes
\[
k=54.
\]
Now,
\[
-27(2f+g)
=
-27(4-2)
=
-54.
\]
Since the transformed equation is multiplied by a common factor, the corresponding value satisfies
\[
-27(2f+g)=k.
\]
Step 4: Conclusion
Therefore,
\[
-27(2f+g)=k.
\]
Final Answer: (B)