I haven’t tested it, but try it this way:
c=comp("expertAssist_After");
ctrl=c.layer("NULL CONTROL").transform.position;
l=c.layer(thisComp.name);
x=ctrl.valueAtTime(time+l.startTime)[1];
k3=ctrl.key(3).value[1];
k4=ctrl.key(4).value[1];
linear(x,k3,k4,0,100)