pklease help sewage in kitchen sink .

my base ment is finished and has a kitchen and laundry room , starting a week ago my washer has been leaking on floor , I notice the drain pipe has been overflowing with usually isnt a issue . the wqasher drains in a 4 foot pipe that is 1 1/2 inch wide, on the floor next to it is a pipe maybe 2 1/2 to 3 1/2 wide that is stopped up with some sort of pressure seal . , when some one flushes the toilet upstairs it backs up in the down stairs sink , so im thinking my main sewer line is clogged ,I went looking for my clean out ????? I found this 50-75 feet away (see pics) the cap was off and there was tp all over and can tell it over flowed at some point! so im thinking i have to snake this baby , here the question , do I snake the clean out ( is that what it is ?) or do i snake from inside the house (that pipe in the floor) or do I do both ?

You have some odd plumbing in that part of the world! Do you know if you have a house trap...or what it is?Start from the closest point the sewer exits the building and check any access point there.If it's caput there ya go.If not make your way back until you have narrowed down the offending line.
Consider calling a professional if you lack proper experience and knowledge.

this is the only pipe outside the house i can find . would you think i should snake the pipe on basement floor then snake this pipe shown in pics? do you think pipe in floor is part of the equation

In most cases, the snakes that are available to us (DIYers) are not capable of clearing a clogged drain. What we end doing is pushing the clog deeper and compacting it, or just pushing through it but not clearing it. It requires a professional plumber with a professional sized auger to do the job. These augers are generally not available at rental shops and even if they were, they are extremely dangerous in the hands of an untrained operator. You should not even consider chemical drain cleaners, they do not work and create a hazard for the plumber who you will eventually have to call.

That is Your clean out. Call a drain co. NEXT