If the symetric difference quotient is given as: (P'(t)= P(t+10)-P(t-10)/20)

Given the info below, how would I compute this equation? I am a little lost and getting a bit frustrated... I am not sure what to use for (P'(t)... Any help would be greatly appreciated...

Code:Date population at t time in years after 1790 1790 3.9 t=0 1800 5.3 t=10 1810 7.2 t=20 1820 9.6 t=30 1830 12.9 t=40

etc

CB