Step 1.  Find the slope of the second line.

         It is  -4.


Step 2.  The slope "m" of the first line must be opposite reciprocal to -4, the slope of the second line.

         It gives you   m = .


Step 3.  Then the equation of the first line must be y = mx + c =   with some constant value of "c" 

         (for now, any value of "c" does fit, and we will choose it later).


         The last equation is equivalent to 4y = x + 4c,   or   x - 4y = -4c.   (*)


         Multiply both sides of the equation (*) by 3 to get an equivalent equation

         3x - 12y = -12c.   Take c =  to get   3x - 12y = 7.


         Now k = -12  is the value you are looking for.