how to give conditon or formula for the following data:

if base <= 2000 calculate base * 0.08

if base >2000 and base <= 3500 calculate base * 0.11

else base >3500 calculate base* 0.15

condition: base <=2000

formula: base * 0.08

condition: base >2000 and base <= 3500

formula: base * 0.11

condition: base >3500

formula: base * 0.15

how can i make codition in only one salary structure