please pake cara ya no 1. bag b bagaimana?

U4 = 24
U9 = 44

U4 = a + (4 -1) b
24 = a + 3b......(1)

U9 = a + (9- 1)b
44 = a + 8b ....(2)

pers (1) dan (2)
a + 3b = 24
a + 8b = 44
____________-
-5b = -20
b= -20/-5
b = 4


a + 3b = 24
a + 3(4) = 24
a + 12 = 24
a = 24 - 12
a = 12

U21 = a + (n - 1)b
U21 = 12 + (21 - 1)4
U21 = 12 + 80
U21 = 92