Use $\displaystyle y = u^k$ to prove $\displaystyle \frac{dy}{dx}= y - (y^4)x$ can be rewritten as $\displaystyle \frac{d}{dx} (e^{3x}u) = 3xe^{3x} $
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Standard Bernoulii diffeq: set $\displaystyle u - y^{-3}$ and see what happens.
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