Assume e^3x + 1 = t

⇒ d(e^3x + 1) = dt

⇒ 3e^3x=dt

⇒ e^3x =

Substituting t and dt in the given equation we get

⇒

⇒

⇒

But t = e^3x + 1

∴ The above equation becomes

⇒ .