There are 2.16 moles of Lithium in 15 grams.
Look at the periodic table to find the molar mass of an element. Molar mass of Lithium is 6.9410 g/mol.
Use dimensional analysis:
15g* (1mol/(6.9410g/mol))=answer
The grams cancel out leaving you with your number in mol units.
Multiply it out and you get 2.16 moles of Lithium.