Let $p_1,p_2,p_3$ be the perpendicular distance between pair of parallel lines represented by $x^2-3x-4=0,y^2-5y+6=0,4x^2+20xy+25y^2=0$ respectively then:

p₁= distance b/w the pair of parallel lines $x^2-3x-4=0$

 $=2\sqrt{\frac{g^2-ac}{a\left(a+b\right)}}=2\sqrt{\frac{\frac{9}{4}+4}{1\left(1+0\right)}}=2\sqrt{\frac{25}{4}}=5$      

 P₂= distance b/w the pair of parallel lines $y^2-5y+6=0$

$=2\sqrt{\frac{\frac{25}{4}-6}{1\left(1+0\right)}}=2\sqrt{\frac{1}{4}}=1$

 P₃= distance b/w the pair of parallel lines $4x^2+20xy+25y^2=0$

$\begin{array}{l}=2\sqrt{\frac{0-0}{4\left(4+25\right)}}=0\\ \therefore p_3<p_2<p_1\end{array}$