Hey there!
A half-life means after a certain amount of time, half of that substance will have decayed after that time.
An equation for exponential decay is the following:
y = a(1 - b)ⁿ
Where a is the original amount, b is the rate of decay, and n is time.
1000 is about 0.3 of a half life, 1000 divided by 138 days (3312 hours) is approximately 0.3.
Let's plug in values to that equation:
y = 321(1 - 0.5)^(0.3)
Simplify.
y = 260.73
There will be 260.73mg of polonium-210 remaining after 1000 hours.
Hope this helps!