Just substitute A into your formula.

y(t) = Answer you're looking for

yo = A

t = 10^9

h = 4.5 x 10^9

b.

Take your formula again. This time,

y(1) = amount left after 1 year

yo = 6 x 10^23

t = 1

h = 4.5 x 10^9

and

y(2) = amount left after 2 years

yo = 6 x 10^23

t = 2

h = 4.5 x 10^9

Number of atoms decayed = Initial atoms - Atoms left.

EDIT: Typo in the second part of b. I put after 1 year again whilst it should be 2 years.