Bitmap from RGB to graysclae

Question

I need to convert pictureBox1.Image from RGB to Grayscale I'm uses this x=(byte)((cc.B*0.28+cc.G*0.66+cc.R*0.51)/(0.28+0.66+0.51)); cc=Color.FromArgb(x,x,x); but it doesn't correct. Sorry for my bed English