bagaimana caranya?,tolong jelaskan?

U3 + U5 = 30
a + 2b + a + 4b = 30
2a + 6b = 30
a + 3b = 15
a = 15 - 3b

U8 = 31
a + 7b = 31
15 - 3b + 7b = 31
15 + 4b = 31
4b = 16
b = 4

jawaban B
semoga membantu
ㅡ