Algebra Help: Radioactive Decay

Final exam tomorrow and I spent the last hr trying to figure out how to solve this by looking at book, notes and online sources, but no luck.
A particular radioactive isotope has a half-life of 1500 years. If we start with 10 grams of the isotope, how much will remain after 1000 yrs?
Ive been given the formula for expontential decay as follows:
y(t)= a e^bt
Please dont just give me the answer. I have the answer but just dont get it. Please explain the process. And if you have a better formula thats easier to follow feel free to share

MUCH better formula is y(t) = a*0.5^(t/h) where h is the half-life

y(1000) = 10*0.5^(1000/1500) = 6.2996 g <---------

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if you are COMPELLED to use y(t) = ae^bt
y(t)/a = e^bt
for half-life,
0.5 = e^1500b
b = ln 0.5/1500 = -0.0004621
y(1000) = 10e^(-0.4621) = 6.2996 g

compare with one line ans above !