The general solution of the differential equation

$(3 y-7 x+7) d x+(7 y-3 x+3) d y=0$ is

The general solution of the differential equation $(3 y-7 x+7) d x+(7 y-3 x+3) d y=0$ is

The general solution of the differential equation $x \cos \frac{y}{x}(y d x+x d y)=y \sin \frac{y}{x}(x d y-y d x)$ is

If the family of curves $y=a e^{4 x}+b e^{-x}$, where $a, b$ are arbitrary constants represents the general solution of the differential equation

$$ f\left(x, y \frac{d y}{d x}, \frac{d^2 y}{d x^2}\right)=0, \text { then } \frac{d f}{d x}= $$