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...

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