Theres 3 questions that I dont get...Please help!! Thanks

1)An xray beam of intensity, I0, in passing through absorbing material x millimeters thick merges with an intensity, I, given by I = I0e^-kx. when the material is 9 millimeters thick 50% of the intensity lost.(I0 = initial amount)

a) calculate the value of the constant k to the three decimal places.

b) What percentage intensity, to one decimal place, remains if the material is 20 millimeters thick?

2) A lab technician placed a baterial cell intovial at 5 a.m. the cells divide in such a way that the number of ells doubles every 4 minutes. the vial is full one hour later.

a) how long does it take for the cells to divide to 4096?

b)at what time is the vial half full?

c) at what time is the vial full?

3)Determine the pH, tothe nearest tenth, of an acidic solution whose [OH-] concentration is 3.98x10^-10 mol/L if [H+][OH-]=1.0 x 10^-14. If the ph of a solution is defined as pH=-log(h+)?

MANY THANKS