Tolong kasih tau caranya ya. terimakasih..

a = 35
b = 4
n = 3 minggu = 3 × 7 = 21
S21 = 21/2 ( 2×35 + (20)×4 )
= 21/2 ( 70 + 80 )
= 21/2 ( 150)
= 1575
^ ^