[tex]a_2 + 3r = a_5 \\ 3 + 3r = 12 \\ 3r = 9 | \div 3 \\ \boxed{r = 3}[/tex]
[tex]a_1 + r = a_2 \\ a_1 + 3 = 3 \\ \boxed{a_1 = 0}[/tex]
[tex]a_5 + 2r = a_7 \\ 12 + 2 \times 3 = a_7 \\ 12 + 6 = a_7 \\ \boxed{ a_7 = 18}[/tex]
[tex]Sn = \frac{a_1 + a_n}{2} \times n [/tex]
[tex]S_7= \frac{0 + a_7}{2} \times 7 \\ S_7= \frac{0 + 18}{2} \times 7 \\ S_7= \frac{18}{2} \times 7 \\ S_7= 9 \times 7 \\ \boxed{ S_7= 63}[/tex]
