Dr. Brown has a radioisotope source containing 1.2 x 106 (also written as 1.2E 06) atoms of Plutonium-238 in 1985, how many atoms will remain in 352 years?

What's the half-life of Plutonium-238?

Once you know that, what's 352 years in terms of half-lives?

Just take 1/2 to that power and multiply it by the number of atoms you started with (i.e. raise 2 to that power and then divide the starting number of atoms by that amount).