offsetting objects

This is probably a stupid question, but I am really terrible with math and haven't been able to figure this out. I am writing a side scrolling game that needs to offset objects with scrolling. When a player shoots a bullet that uses sin/cos for an angled shot, how would I go about moving this bullet backwards as the player runs forward? I've tried changing the X, Y origin, but this changes the whole path of the bullet.

Here is example code (not bmax because im lazy)

Graphics 640, 480, 16, 2
SetBuffer BackBuffer()

A = 290: R = 0
X = 320: Y = 240

While Not KeyDown(1)

Rect R * Cos(A) + X, R * Sin(A) + Y, 4, 4
R = R + 3
;X = X - 1

Flip:Cls
Wend:End

If you uncomment the X line, the bullet path changes instead of moving backwards while still traveling at the same angle. How do I accomplish this?