Extract world location from modelview matrix

I am using this api to develop an AR game.In this game, I use a tracker (Tracker A)  to determine a battle field and some game unit,I have another tracker (Tracker B) to determine another game unit and all of the unit are not do any translation.Tracker A- multi unit- translationTracker B- single unit- no translationNow, i want to calculate the distance between tracker A unit and tracker B unit.But the API just return the modelview matrix and projection matrix of the tracker A and tracker B.I don't know how to get the world location of these units.I try to use following method to get the distance between tracker A and tracker B:Math.sqrt(    Math.pow(trackerA.modelViewMatrix[12]- trackerB.modelViewMatrix[12], 2) +    Math.pow(trackerA.modelViewMatrix[13]- trackerB.modelViewMatrix[13], 2) +    Math.pow(trackerA.modelViewMatrix[14]- trackerB.modelViewMatrix[14], 2))I try to move the tracker B to test it and it seem ok, but i am not sure is it correct.I suppose above method is correct, but i cannot get the distance between tracker A unit and tracker B unit.Because the tracker A unit had been translate.Is it possible to get the world location of the game unit?