You can model the part at 90 degrees, and then use the Move Face tool to rotate the feature about the top connecting edge.
N.B. You can select features (as well as single faces) when picking which faces to move.
This whole model can be made with four features, and no additional planes or reference geometry beyond the base planes.
- an extrude from above for the 'L' bracket
- A cut from the front to add slots and to round off the end.
- An extrude from the front for the 'notched' plate, shown blue in the image below.
- Move face, to rotate it into position, shown yellow in the image below.
Again, the key point to take away from this is editability. You should be able to change any one dimension without the rest of the model needing to be manually updated. Selecting the Extrude feature (made in step 3) as the input for the Move face (step 4) means that even if you edit the extrude to have extra holes or to remove a slot, the move face will still work. Manually selecting the faces may cause this feature to fail as new faces appear or old faces are removed.