Take into account that two lines are parallel if their slopes are equal.
The general form of th equation of a line is:
y = mx + b
where m is the slope and b the y-intercept.
You can notice that for the following lines:
y = -4x - 1
y = -1/4x + 8
the slopes are not equal.
The lines are perpendicular if you verify the following:
m1 = -1/m2