Are lines y=-x-4 and 5x+5y=20 perpendicular?

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