Equation of normal at #x=x_0# on the curve #y=f(x)# is perpendicular to the tangent at #x=x_0# at #f(x)#.

As slope of tangent at #x=x_0# on the curve #f(x)#, is given by #f'(x_0)#, slope of normal is #-1/(f(x_0))# and equation of normal is

#y=-1/(f(x_0))(x-x_0)+f(x_0)#

Here, we have #y=f(x)=-sqrt((x+1)(x+3))#

and #f'(x)=(df)/(dx)# - using chain rule

= #-1xx1/(2sqrt((x+1)(x+3)))[1xx(x+3)+1xx(x+1)]#

= #-1/(2sqrt((x+1)(x+3)))(2x+4)=-(x+2)/sqrt((x+1)(x+3))#

and while #f(0)=-sqrt3#, #f'(0)=-2/sqrt3#

as such slope of normal at #x=0# is #sqrt3/2# and

equation of normal is #y=sqrt3/2x-sqrt3#

graph{(y-sqrt3/2x+sqrt3)(y+sqrt((x+1)(x+3)))=0 [-12.92, 7.08, -8.52, 1.48]}

The curve or normal is not appearing in the interval #(-1,-3)# as #f(x)=-sqrt((x+1)(x+3))# is not defined in the interval #(-1,-3)#.