Check it out:
Take the 2 ... Write it as a fraction:
Flip it over: ... and change the sign:
Let's do a problem:
Let's find the equation of the line that passes through the point  
( 1 , 3 ) and is perpendicular to the line  . 
What do we need?
a point

a slope

STEP 1: Find the slope
It's perpendicular to  ... 
So, we'll get this guy's slope ... flip it and change the sign! Easy!