The differential equation $y\frac{dy}{dx}+x=a$ (a is any constant) represents 

We have,

$y\frac{dy}{dx}+x=a\Rightarrow ydy+xdx=adx$

Integrating, we get

$\frac{y^2}{2}+\frac{x^2}{2}=ax+C\Rightarrow x^2+y^2-2ax+2C=0$

which represents a set of circles having centre on x-axis