Not sure if I’d have gotten this one without Cavy’s hint, I’m pretty bad with differential equations

Let z = ln(y)

Then z = dy/dx / y

dy/dx / y is the derivative of ln(y)

So we have z = d/dx ln(y), or ln(y) = d/dx ln(y)

ln(y) is its own derivative - so we must have ln(y) = e^x, which implies y = e^(e^x)