tolong bantuan jawaban + cara kak. plisss

Diketahui :
U3 + U12 = 46
U-21 = 77

Ditanya : U51..?

Jawab :
U3 = a+(n-1)b = a+(3-1)b --> a+2b
U12 = a+(n-1)b = a+(12-1)b --> a+11b

Jadi, U3 + U12 = 46
=> a+2b+a+11b = 46
2a+13b = 46... pers. 1

U21 = 77
=> a+(21-1)b = 77
a+20b = 77... pers. 2

Eliminasi pers. 1 dan 2
2a + 13b = 46
a + 20b = 77
--> 2a + 13b = 46
2a + 40b = 154
-27b = -108
b = 4
--> a+20 x 4 = 77
a + 80 = 77
a = -3

Jadi, U51 = a+(n-1)b
= -3 + (51-1)4
= -3 + 50 . 4
= -3 + 200
= 197