C++ Project #3 Need Help With Formula

V = (D)(W - (2D))(L - (2D)) ...expand
  = 4(D^3) - 2W(D^2) - 2L(D^2) + WLD   ...refactorize a section:
  = 4(D^3) - 2(W + L)(D^2) + WLD

dV/dD = 12(D^2) - 4(W + L)D + WL

    0 = 12(D^2) - 4(W + L)D + W
    
      = 12(D^2) - (4L + 4W)D + WL

    x = (-b +- sqrt((b^2) - 4(ac))) / 2a

    a = 12
    b = -(4L + 4W)
    c = WL
therefore:

    D = ((4L + 4W) + sqrt((-(4L + 4W)^2) - 4(12WL)))) / 24

or  D = ((4L + 4W) - sqrt((-(4L + 4W)^2) - 4(12WL)))) / 24

#include <math.h>

#define quadA(a, b, c) (float)((-(b) + sqrt(((b) * (b)) - (4 *((a) * (c))))) / ( 2 * a)) //quadratic equation (addition)
#define quadS(a, b, c) (float)((-(b) - sqrt(((b) * (b)) - (4 *((a) * (c))))) / ( 2 * a)) //quadratic equation (subtraction)
#define vol(d,w,l) (float)((d) * (w) * (l)) //simple volume macro

float OptimizeDepth(float length, float width)
{
    float L = length; //convenient shorthand
    float W = width; //...

    float DA, DS; //hold the depths
    float VA, VS; //hold the volumes

    DA = quadA(12.0f, -(4*L + 4*W), W*L);
    DS = quadS(12.0f, -(4*L + 4*W), W*L);

    VA = vol(DA, W - (2*DA), L - (2*DA));
    VS = vol(DS, W - (2*DS), L - (2*DS));

    return ( VA > VS ? DA : DS );
}