Answer
See below.
Work Step by Step
a) Our equation is: $9000=18700\cdot0.84^t\\\frac{9000}{18700}=0.84^t\\t=\log_{0.84}{\frac{9000}{18700}}\approx4.19$
b) Our equation is: $6000=18700\cdot0.84^t\\\frac{6000}{18700}=0.84^t\\t=\log_{0.84}{\frac{6000}{18700}}\approx6.52$
c) Our equation is: $2000=18700\cdot0.84^t\\\frac{2000}{18700}=0.84^t\\t=\log_{0.84}{\frac{2000}{18700}}\approx12.82$