A line in point-slope form has the equation
y = mx + b
where m=slope (increase in y for unit increase in x), and
b=y-intercept (value of y where line cuts y-axis)
The original line is
y=(-1/2)x + 11
so
slope = m = -1/2
Any line perpendicular to a line with slope m has a slope of m1=-1/m
So the slope m1 of the required line
m1 = -1 / (-1/2) = +2
and the required line therefore has an equation of
y=2x+b
Knowing that the line passes through P1=(x1, y1)=(4,-8), we can find b using the point slope form of a line with slope m : (y-y1) = m(x-x1)
where m=+2 as found above.
Substituting values, m=+2, x1=4, y1= -8
y-(-8) = +2(x--4)
simplify
y+8 = 2x-8
=>
y=2x-16 (in point slope form)
