Spawning object help

So im trying to spawn an object directly behind the player using Lua (regardless of the direction the player is facing) I'm using the function

P_SpawnMobj(player.mo.x, player.mo.y, player.mo.z + 15*FRACUNIT, MT_"insertobjectnamehere")

although...i dont know how to make is spawn..behind the player like i need it to, any help would be much appreciated.

To convert a desired angle and distance into x and y positions, use the formulae x = FixedMul(d, cos(a)) and y = FixedMul(d, sin(a)), where a is the angle in ANGLE_X format and d is the distance in FRACUNIT scale. In this case, you can write

local a = player.mo.angle + ANGLE_180
local d = <how far behind the player you want the object to spawn>
P_SpawnMobj(player.mo.x + FixedMul(d, cos(a)), player.mo.y + FixedMul(d, sin(a)), player.mo.z + 15*FRACUNIT, MT_"insertobjectnamehere")