Hey I've been trying this problem forever and it's not working if I could have some help that would be Awsome

Strontium-90 has a half-life of 28 days. (a) A sample has initial mass of 50 mg. Find a formula for the mass remaining after t days, y(t). y(t) = (b) Find the mass (in mg) remaining after 35 days. (Round your answer to one decimal place.) mg (c) How long does it take (in days) the sample to decay to a mass of 2 mg? (Round your answer to the nearest day.) days