Well the slope of a perpendicular line is the other slope flipped upside down and the sign changed so m = -3
Then you put the info you have into the equation
y - y1 = m(x-x1)
Y - (-8) = -3(x-5)
Y + 8= -3x + 15
Y = -3x + 7
And you can leave it like that or change the order of the numbers if you'd like! (Don't forget to change the sign if you move something over the = sign!! )