I'm having some trouble using the easing equations suggested by Robert Penner for ActionScript (http://www.robertpenner.com/easing/, and a Flash demo here) in my C# XNA game. Firstly, what is the definition of the following variables passed in as arguments to each equation?

float t, float b, float c, float d

I'm currently calculating the new X position of a sprite in the Update() loop, however even for the linear tween equation I'm getting some odd results. I'm using the following values:

float t = gameTime.TotalGameTime.TotalMilliseconds;
float d = 8000f;
float b = x.Position.X;
float c = (ScreenManager.Game.GraphicsDevice.Viewport.Width >> 1) - (x.Position.X + x.frameSize.X / 2);

And this equation for linear easing:

 float val = c*t/d + b;