I need a little clarification about how to use this equation.

f'(x)=f(x+h)-f(x-h)/2h + O(h^2)

here is the info

time___ 0 20 40 . . . . . . . . . . . .......................... . . 160 180 200

altitude 60 10170 33835 ........................107892 119626 162095

find the central difference at t=20 and t=180

I know how to find h but how to i put all these x values into the equation?