Horton equation:

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

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

3.5 - 1.2 = (f_o - 1.2) e^-k

2.1 - 1.2 = (f_o - 1.2) e^-2k

2.3 = (f_o - 1.2) e^-k

0.9 = (f_o - 1.2) e^-2k

Dividing these two equations:

2.555 = e^k

k = ln (2.555) = 0.938 hr^-1    ANSWER.

Solving for f_o:

f_o = 2.3 e^k + 1.2

f_o = 7.076 mm/hr    ANSWER.

f_c = 1.2 mm/hr    ANSWER.

Verification:

f = 1.2 + (7.076 - 1.2) e^-0.938 = 3.5 mm/hr   OK

f = 1.2 + (7.076 - 1.2) e^{-2 × 0.938} = 2.1 mm/hr   OK