tolong bantu jawab aku gak ngerti:')

U5 -----> a + 4b = 13
U9 -----> a + 8b = 21
             ----------------- -
                  - 4b = -8
                      b = 2
a + 4b = 13
a +4(2) = 13
a = 13 - 8
   = 5

a) beda = b = 2

b) Un = a + (n - 1) b
    U10 = 5 + (10 -1 )2
            = 5 + (9)2
            = 23

c) U11 = 5 + (10)2
           = 25
    Sn = n/2 (a + Un)
    S11 = 11/2 (5 + 25)
           = 165