I need help with this asap. Thank you

A sample of 250 grams of a radioactive substance decays exponentially. The half-life of the substance is 42 years. (a) Determine a model of the form A(t) = Age" to represent the amount of the substance remaining in the sample after t years. Round your value for k to four decimal places. (b) Use your equation from part (a) to determine how many grams will be remaining after 100 years. Round to the nearest tenth of a gram