Answer: Hello!
the information that we have is:
Heather bought enough shells to make x necklace.
Each necklace needs 16 shells.
Heather has made 10 necklaces.
If heater bought enough shells to make x necklaces, and she already did make 10 necklaces, then she can make another (x - 10) necklaces.
this (x - 10) necklaces need 16 shells each, then she has a total of:
(x -10)*16 shells left.
then the given equation "16x+10" is not a reasonable way to represent the number of shells that Heather has left to make necklaces.