Bantuin kak sama cara dapetnya gimana

Deret aritmatika
Un = a + (n-1)b

U3 + U7 = 28
(a + 2b) + (a + 6b) = 28
2a + 8b = 28
a + 4b = 14

b = 3 , maka
a+ 4b = 14
a + 4.3 = 14
a +12 = 14
a = 2

U7 = a + (7-1)b
U7 = a + 6b
U7 = 2 + 6.3
U7 = 2 + 18
U7 = 20 (B)

smg membantu