The slope formula (y1-y2)/(x1-x2) won't do you any good here.
[But you should learn that numerators and denominators that 
contain more than one number or one letter MUST be enclosed
withing parentheses to tell where the numerators and denominators
begin and end.]

You have to get the slope from this given line 

y = 4x + 3

Its slope of it is 4 because you compare it to y = mx + b and m = 4

A line that is perpendicular to a line with slope 4 has slope formed by:

1. Taking the reciprocal of 4, which is 1/4.  That's because a "run" becomes
   a "rise" and vice-versa.
2. Changing the sign to -1/4 because if a line goes uphill to the right, a 
   line perpendicular to it will go downhill, and vice-versa.

So the slope of the desired line is -1/4.  Now you can substitute m = -1/4
and (x1,y1) = (-4, -5)

  y - y1 = m(x - x1)
y - (-5) = -1/4(x - (-4))
   y + 5 = -1/4(x + 4)
   y + 5 = -1/4x - 1
       y = -1/4x - 6

Edwin