the lines consider perpendicular if (the slope of line 1) * (the slope of line 2) = -1
so get the slope of the lines ,1st put the equation on the form of y = mx+c
where the coefficient of x (m) is the slope
y=-x-4 .... m1= -1
5x+5y=20
y=-x+4 .... m2 = -1
so m1 * m2 = -1 * -1 = 1
then, the two lines aren't perpendicular
