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