Step 1: Converting the equation.
Rewriting the given equation:
\[
\frac{dy}{dx} = e^{3x + 4y}
\]
Separating variables:
\[
e^{-4y} dy = e^{3x} dx
\]
Integrating both sides:
\[
\int e^{-4y} dy = \int e^{3x} dx
\]
\[
\frac{e^{-4y}}{-4} = \frac{e^{3x}}{3} + C
\]
Multiplying by -4:
\[
e^{-4y} = -\frac{4}{3} e^{3x} + C
\]
Using \( y(0) = 0 \), solving for \( C \), we get:
\[
4e^{3x} + 3e^{-4y} = 7
\]
Thus, the correct answer is (D).