The sum of n terms of a geometric sequence with first term "a" and common ratio "r" is given by [tex]S_{n}=a\dfrac{r^{n}-1}{r-1}[/tex] You have a=21, r=3, so the sum of 10 terms is [tex]S_{10}=21\cdot \dfrac{3^{10}-1}{3-1}=620004[/tex]
It’ll be 620,004 Each number is multiplied by 3 so the 10 numbers would be 21; 63; 189; 567; 1,701; 5,103; 15,309; 45,927; 137,781; 413,343 Adding those numbers equals 620,004
