Urgent Algebra homework help needed

1. The half-life of carbon-14 is 5700 years. If you start with 10 grams of carbon-14, how much will remain in 22,800 years?

2. A light-year is approximately 5.88 x 10 to 12 power miles. Estimate the distance from Earth to a star so far away that its light takes 9.4 years to reach us.

The answer to #1 is .625g and the answer to #2 is 5.55 x 10 to the 13th power, but I don't know how to work the problems.

Both of these problems are suppose to use exponents and I have a test on similar type questions tomorrow. :(