A specific radioactive substance follows a continuous exponential decay model. It has a half-life of 21

hours. At the start of the experiment, 63.2g is present.

(a)Let t be the time (in hours) since the start of the experiment, and let y be the amount of the substance at time.

Write a formula relating y to x. Use exact expressions to fill in the missing parts of the formula. Do not use approximations.