draw a horizontal and vertical construction lines at the midpoints (should snap there in Fusion) then draw a point at the intersection. do the same thing for both shapes. Then do a copy (Move) point to point and select the two center points. Whole adventure should take under a minute.
While @maleybr 's method is perfect for dealing with this scenario,“after the fact” . The real “easy way” is start off your sketch in a way the sets you off in the right direction.
When the OG sketch was created a "center rectangle " should have been used and started on the origin point. this would simplify all future edits and give you a great center to snap to or create from.
When working symmetrical and semi symmetrical designs starting at origin prompts great workflow.
this screen grab shows the 2 places where the center point rectangle can be selected from and it also points to the origin 's visibility toggle.
@TinWhisperer Thank you, this is good to know, and makes a lot of sense.
When I originally started watching videos on how to use Fusion, most (if not all) videos basically said that “It does not matter where you place “something”, because it can be changed later”
I am of course paraphrasing, but now I see that starting off at the center makes a difference.