Horton equation:

f = f_c + (f_o - f_c) e^-kt

f - f_c = (f_o - f_c) e^-kt

2.5 - 0.7 = (f_o - 0.7) e^-k

1.5 - 0.7 = (f_o - 0.7) e^-2k

1.8 = (f_o - 0.7) e^-k

0.8 = (f_o - 0.7) e^-2k

Dividing these two equations:

2.25 = e^k

k = ln (2.25) = 0.81093 hr^-1    ANSWER.

Solving for f_o:

f_o = 1.8 e^k + 0.7

f_o = (1.8 × 2.25) + 0.7 = 4.75 mm/hr    ANSWER.

f_c = 0.7 mm/hr    ANSWER.

Verification:

f = 0.7 + (4.75 - 0.7) e^-0.81093 = 2.5 mm/hr   OK

f = 0.7 + (4.75 - 0.7) e^{-2 × 0.81093} = 1.5 mm/hr   OK