n = 0.8d + 53
n → Total number of necklaces she owns
d → Amount of money she spends on additional necklaces
You want to know what "d" would be equal to if "n" is equal to 73.
Replace the "n" in the equation with 73 and isolate "d" :
73 = 0.8d + 53
subtract 53 from each sides
20 = 0.8d
Divide by 0.8 on each sides
25 = d
Since "d" is equal to 25 when "n" is equal to 73, this means that for Tracy to own a total of 73 necklaces, she needs to spend 25$ on new necklaces.
There you go! I really hope this helped, if there's anything just let me know! :)