Mirror x-position and time-delay expression

Using Limber, I want to mirror the right arm-movement ("B-Wrist") for the left arm ("F-Wrist") and delay it with 17 frames. For the y-position on the left arm, I use this expression which works how I want: 

A = thisComp.layer("B-Wrist").transform.yPosition

F = 17; //frames to delay

T = time - (F *thisComp.frameDuration);

A.valueAtTime(T);

On the x-position, it's using the right arms x-position and previously I've added a "*-1" after the parent-expression when not delaying the movement. But how do I use the expression above and add that negative mirror expression?! 

Thanks