Please help. My teacher never went over crystal lattices and the book doesn't cover it either :(

Question

Copper crystallizes in a face-centered cubic lattice. If the edge of the unit cell is 351pm, what is the radius of the copper atom?

Answer 1

4r = a*2^1/2
a is the edge length, r is radius.

Answer 2

r= a/2✅2(2 root 2)where be the edge length