Solve: `(dy)/(dx)-2y=cos3x`

Explanation

To solve the differential equation \(\frac{dy}{dx} - 2y = \cos(3x)\), we will follow the steps for solving a first-order linear differential equation.

### Step 1: Identify the standard form
The given equation is already in the standard form:
\[ \frac{dy}{dx} + P(x)y = Q(x) \]
where \(P(x) = -2\) and \(Q(x) = \cos(3x)\).
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