tolong bantu ya kawan

U9 = 36 U21 = 84 Un = a+(n-1)b U100 = ?
a+8b = 36 a+20b = 84
cari "b"nya
84 = a+8b+12b
84 = 36+12b
12b = 48
b = 4

cari "a"nya
a+8b = 36
a+8(4) = 36
a+32 = 36
a = 4

U100 = 4+(100-1)4
=4+99 4
=4+396
U100 = 400