Final answer:
By subtracting the number of Tomer's crystals (7) from Lehie's collection (21), it can be determined that Lehie has 14 more crystals than Tomer.
Step-by-step explanation:
To determine how many more crystals Lehie has than Tomer, we need to subtract the number of Tomer's crystals from the number of Lehie's crystals. Lehie has a collection of 21 crystals, while Tomer has 7 crystals. The difference between their collections can be found using the subtraction operation:
Lehie's crystals - Tomer's crystals = Difference
21 - 7 = 14
Therefore, Lehie has 14 more crystals than Tomer in her collection.