Help please

[JAZZCNC]

Create a plane at the angle then create sketch or extrude on this plane using the convert entity's intersection tool.

To create the plane draw a sketch on the front plane with line at the angle and location you want. See pic1
Then create plane from Reference geometry option using the angled line as first reference and front plane as second. See pic2

Next select the new plane and create sketch on it. Use the Convert entity's tool and select the intersection tool. see pic3
Then you can offset the result and extrude.