tolong dong nomor 1 sama 2 itu gimana, tolong ya. itu pake jalan ngisinya. besok dikumpul help me

1)
U6 = 20
b = 3
a =......??

Un = a + (n-1)b
U6 = a + (6-1)b
20 = a + (5)3
20 = a + 15
a = 20-15
a = 5

2)
U5 = 8
U20 = 68
U36=...??

U5= a + (4)b
8 = a + 4b
U20 = a + (19)b
68 = a + 19b

a+4b=8
a+19b=68_
-15b = -60
b = -60/-15
b = 4

a + 4(4) = 8
a + 16 = 8
a = 8-16
a = -8

U36 = a + (35)b
U36 = -8 + (35)4
U36 = -8 + 140
U36 = 132