This like of code seems to give the correct result in Filter-more exercise in CS50.

int r = round(sqrt(pow(rgx, 2) + pow(rgy, 2)));
int g = round(sqrt(pow(ggx, 2) + pow(ggy, 2)));
int b = round(sqrt(pow(bgx, 2) + pow(bgy, 2)));
            if (r > 255)
            {
                r = 255;
            }
            if (g > 255)
            {
                g = 255;
            }
            if (b > 255)
            {
                b = 255;
            }
            copy[i][j].rgbtRed = r;
            copy[i][j].rgbtGreen = g;
            copy[i][j].rgbtBlue = b;

But this code doesnt seem to work. Seems to Me that both codes are very similar.

copy[i][j].rgbtRed = round(sqrt(pow(rgx, 2) + pow(rgy, 2)));
copy[i][j].rgbtGreen = round(sqrt(pow(ggx, 2) + pow(ggy, 2)));
copy[i][j].rgbtBlue = round(sqrt(pow(bgx, 2) + pow(bgy, 2)));
            if (copy[i][j].rgbtRed > 255)
            {
                copy[i][j].rgbtRed = 255;
            }
            if (copy[i][j].rgbtGreen > 255)
            {
                copy[i][j].rgbtGreen = 255;
            }
            if (copy[i][j].rgbtBlue > 255)
            {
                copy[i][j].rgbtBlue = 255;
            }