We have,

T_n = (-4n + 15)

T₁ = -4 + 15 = 11

T₂ = -4 * 2 + 15 = 7

T₃ = -4 * 3 + 15 = 3

The first term is 11 and the common difference is, d = T₂ – T₁ = 7 – 11 = -4

The sum of first 20 terms, S₂₀ = [2 * 11 + (20 – 1) (-4)]

= 10 (22 – 76) = -540