I can't cut perfect rectangles

Hello, again I am here asking for some help with the tuning of this little gem.
Everything in the machine works great except for one thing, both horizontal and vertical straight lines are somewhat inclined. I can’t cut perfect rectangles. Which may be due? I’m thinking of gantry square .


On the one hand, the vertical, the problem lay in the x-axis bar. It was misaligned. Bad horizontal lines are due to bad tension on the x-axis belt. I thought this was very tight but I was wrong. Right now I still can’t tighten the belt well. I cannot tighten one of the screws well and that side of the belt is still less tense than the other. I am about to break the screw. Can someone tell me some good tutorial please, I am unable to find it right now.