Don't know what formula to use Need Help please

Question

The mass, 𝑀, (measured in grams) of radioactive substance remaining after 𝑦 years is given by: 

𝑀 = 1400𝑒−0.03𝑦

1. Find the mass remaining after 10 years.
2. Find the time taken for the mass to reach 10% of its original value. Present your answer to the nearest month.
3. Find the rate of change in the mass after 100 years.

Answer 1

1) plug in y = 10 into the expression M = 1400e^-.03y

2) 1400 is the initial mass (as you can see by subbing y = 0 in)

M/1400 = e^-.03y You want to solve for y given M/1400 = .1

Take ln of both sides: ln(.1) = -.03y and y = ln(.1)/(-.03)

3) Rate of change of M is dM/dy

dM/dy = -.03(1400)e^-.03y (sub in y = 100 years)

Please consider a tutor. Take care.