Question 1175591
<pre>
There are billions of trillions of lines perpendicular to it.

{{{matrix(1,9,
m,""="",(y[2]-y[1])/(x[2]-x[1]),""="",(10-2)/(5-1),""="",8/4,""="",2)}}}

Slope of reciprocal: invert {{{2/1}}} as {{{1/2}}} and change the sign {{{-1/2}}}.  
That's the slope of any line perpendicular to it.

You no doubt have to choose which one from a list.  But since we are
not mind-readers, and you didn't tell us the choices on the list, we 
cannot tell which one of the billions of trillions of possible lines
perpendicular to choose. 

Edwin</pre>