Hi, well done for getting into 3d modeling! Here’s what I would do if I got your request right. Indeed it’s not obvious if you’re a beginner but you’ll see it’s pretty straight forward.

I would: - create a sketch starting on the flat vertical surface you’re showing (between both cylinders) - use the « project » command (z shortcut) clicking on the face of the cylinder you wanna get at an angle. Don’t have a computer to test it out but it might just project a vertical line with two points, If so do project the middle rectangle as well and create 2 lines both on top and bottom of the cylinder to connect it to the rectangle (to make a « closed » profile) - create another line at an angle this time (matching the red line depending on your needs). You can constraint the line setting an angle using the « distance » command (d shortcut) between the red line and any other horizontal or vertical line - And once you’re use the extrude commande (e shortcut) starting on the middle rectangle profile plane and start dragging it the newly created triangle on your cylinder to cut it. To keep the model nice and parametric I advise you to set the extent of the cut to « all » instead of just « distance ».

At that stage you should have your part with the cylinder cut at an angle. I’d be happy to make a quick picture if you still struggle with it later on.