find k , if the 4x-y=1 is perpendicular to the line 5x-ky=2?

if two lines are said to be perpendicular, then the product of their slopes is equal to '-1'. i.e a1X+b1Y=c1 & a2X+b2Y=c2 then their respective slopes are '-a1/b1' and '-a2/b2'. if these two lines are perpendicular then '-a1/b1'*'-a2/b2'=-1