tolong ya menjawab menggunakan cara

U3 = ar^2. U5,= ar^4. U6 = ar^5
U3xU6 = ar^2 x ar^5
= a^2.r^7 = 16
(a.r^4)^2.r^-1 = 16
(8)^2.r^-1 = 162
64.r^-1 = 16
r^-1 = 16/64
r^-1 = 1/4
r= 4
ar^4 = 8
a = 8/r^4
a = 8/256
a = 1/32
S6 = a ( r^6 - 1 ) / ( r - 1 )
= 1/32 ( 4^6 - 1 ) / ( 4 - 1 )
= 1/32 ( 4096 - 1 ) / 3
= 43 16/93

ini ya...
semoga membantu