I suppose you are learning interpolation .... a type of extimation that assumes the graph is linear between two points (2.98 and 3.01 in this case)
  
 from (f(2.98) to f (3.01) is     1.96 - 2.08 =  - .12   
  
    (3.01 - 2.98)  is .03
  
 -.12/.03 = -4      the graph changes   -4    per   1 change in f(x)
  
 for   .01   change (from   2.98 to 2.99)
        it will be   2.08  - 4(.01) = 2.04