soal mtk tentang deret tolong dijawab

b = (57-75)/(10-7) = -18/3 = -6
U7 = a + 6b
75 = a + 6x(-6)
a = 75+36
a = 111

U28 = a + 27b = 111 + 27x(-6) = -(162-111) = -51

111 = -6n
n = 111/-6 = -18½

n = 18+1 = 19
bilangan negatif pertama pada suku ke -19 yaitu -3