Perpendicular lines are lines that, when intersect the original line, form a right angle. This is formed when two lines' gradients are the negative reciprocals of each other.
In essence, [tex]m_1 \cdot m_2 = -1[/tex]
Now let 5 be [tex]m_1[/tex]
Thus, [tex]5 \cdot m_2 = -1[/tex]
[tex]m_2 = -\frac{1}{5}[/tex]
That means any line with a gradient or slope of [tex]-\frac{1}{5}[/tex] is a line that is perpendicular to this line.
In essence, the general equation would be: [tex]y = -\frac{1}{5}x + C, C \in R[/tex]
