Tolong kerjakan dengan benar’ terimakasih

U21?
Un = a+(n-1)b
U5 = 36 => a+4b = 36
U9 =45 => a+ 8b = 45
_________________________ __
-4b = -9
b = 2,25

a+4b = 36
a+4(2,25) = 36
a+9 = 36
a = 27

U21 = a+20b
= 27+20(2,25)
= 27+45
= 72