Concept:
• Variable Separable Method: Group all \( y \) terms with \( dy \) and \( x \) terms with \( dx \).
• Use exponent laws: \( e^{a-b} = \frac{e^a}{e^b} \).
Step 1: Separate the variables
Given: \( \frac{dy}{dx} = e^{3x} \cdot e^{-y} \)
\[ \frac{dy}{dx} = \frac{e^{3x}}{e^y} \]
Multiplying both sides by \( e^y \, dx \):
\[ e^y \, dy = e^{3x} \, dx \]
Step 2: Integrate both sides
\[ \int e^y \, dy = \int e^{3x} \, dx \]
Applying standard integration formulas:
\[ e^y = \frac{e^{3x}}{3} + C_1 \]
where \( C_1 \) is the constant of integration.
Step 3: Simplify the expression to match options
Multiply the entire equation by \( 3 \):
\[ 3e^y = e^{3x} + 3C_1 \]
Let \( 3C_1 = C \) (a new constant):
\[ 3e^y = e^{3x} + C \]
This matches option (A).