Trophy chain adjusment

If it's the same as my Trident i.e. an eccentric adjuster, then the slack in the chain at its tightest point when measured in the middle should be 35-40 mm. Slacken the pinch bolts at each end of the swinging arm and adjust as necessary using the eccentric adjusters on either side of the s/a. A suitably sized allen key is required for this procedure. Check the markers on each side of the s/a are in the same position to keep the wheels in line. A suitable long straight edge from back wheel to front is the best way to check this, but mine has always been ok just using the marks on the eccentric.
If the bike is new to you, I would also clean and oil the chain.