General equation of the straight line is [tex]y=mx+c[/tex]. Characteristic of perpendicular straight lines slopes: [tex]m_1\cdot m_2=-1[/tex]. Now we can find the lope of our equation: $$m\cdot 4 = -1, \ \ m=-\frac{1}{4}$$ If [tex]m=-\frac{1}{4}[/tex], then we have [tex]y=-\frac{1}{4}x+c[/tex]. Now we can easy find c by inserting given point coordinates to our equation. $$5=-\frac{1}{4} \cdot 8 + c, \ \ c=7$$ Answer: [tex]y=-\frac{1}{4}x+7[/tex]