The gradient of the given line is 2 (coefficient of x), so the gradient of the perpendicular is -½.
The equation of the perpendicular is y=-½x+a. To find a we plug in the given point:
-5=+3/2+a, so a=-5-3/2=-13/2 and y=-½(x+13).
Another way to write this is in the form y-Y=-½(x-X) where (X,Y)=(-3,-5) the given point.
So y+5=-½(x+3), which is equivalent to y=-½(x+13).