Let the a be first term and d be common difference of an AP

As n = 37 (odd)

Middle term will be

Three middle most terms will be

18^th, 19^th and 20^th terms

Given,

a₁₈ + a₁₉ + a₂₀ = 225

a + 17d + a + 18d + a + 19d = 225 [ using a_n = a + (n - 1)d]

3a + 54d = 225

3a = 225 - 54d

a = 75 - 18d [ eqn 1]

Also last three terms will be 35^th , 36^th and 37^th terms

Given,
a₃₅ + a₃₆ + a₃₇ = 429

a + 34d + a + 35d + a + 36d = 429

3a + 105d = 429

a + 35d = 143

75 - 18d + 35d = 143 [ using eqn1]

17d = 68

d = 4

using this value in eqn 1

a = 75 - 18(4)

a = 3

so AP is a, a + d, a + 2d….

i.e. 3, 7, 11….