parallel: 1
perpendicular: 2, 4
neither: 3
two linear functions f1(x) = a1 * x + b1 and f2(x) = a2 * x + b2, are:
1/ parallel - if a1 = a2
2/ perpendicular - if a1*a2 = -1
-x + 4y = 20
4y = x + 20 /:4
y = 1/4 x + 5 //slope of the line -x + 4y = 20 is 1/4
