SOLUTION: The line I goes through the points (1,2) and (5,10). What could be the equation of a line perpendicular to I? Thanks!

There are billions of trillions of lines perpendicular to it.



Slope of reciprocal: invert  as  and change the sign .  
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