If m and n are respectively, the order and degree of the differential equation of the family of parabolas with origin as its focus and X-axis as its axis, then $mn-m+n=$

The differential equation of the given family of parabolas 
 $y^2=4a(x+a)$
Differentiate w.r.t x
$\begin{array}{l}2y{y}_1=4a\\ y^2=2y{y}_1\left(x+\frac{y{y}_1}{2}\right)\\ \Rightarrow y=2x{y}_1+y{y}_1^2\\ \Rightarrow y{\left(\frac{dy}{dx}\right)}^2+2x\frac{dy}{dx}=y\end{array}$
Order = m = 1
Degree = n = 2
mn-m+n = 3