Consider the line x + 3y= -9 what is the slope of a line parallel to this line? what is the slope of a line perpendicular to this line?

Hello ther is a solution :
 x+3y = -9
3y =-x-9
y = (-1/3)x-3  
1 )  the slope of a line parallel to this line is : (-1/3)
2 ) let  '  a '   the slope of a line perpendicular a line: x+3y = -9
I'have  (-1/3)× a = -1
a = 3

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