drainage problem

[arnet]

Wondering what is causing a toilet that is in a powder room next to the laundry room to empty out everytime the washer is draining into the sink. The laundry tub removes the water very quickly but it is interferring with the toilet. When the toilet is flushed it almost comes up to the rim after being at an extremely low point. Wondering if I have some kind of a blockage problem in the line from the laundry tub to the toilet. What else could be causing this to happen. No other drains are slow and there is no problem with the sink in the powder room. Help before I call a "Pro"

[HayZee518]

sounds to me there is a blockage in the stack of the toilet waste line. usually but not all the time the toilet drains into a T-Y fitting which goes up the wall to the roof stack - same size as the outlet - most times the washer or sink outlet goes into a side inlet on the toilet drain - 2" diameter pipe - within six feet of the waste pipe. longer needs a separate air line piped into and above the toilet waste arm to eliminate the vacuum created when either is flushed or emptied.