tolong caranya gimana

U4 = a + 3b = 24
U10= a + 9b = 60
- 6b = - 36
b = 5
a + 3b = 24
a + 3(5) = 24
a + 15 = 24
a = 24 - 15
a = 9

U15 = a + (n-1)b
= 9 + (15-1)5
= 9 + 70
= 79