a3=7
a9=18
a3=a1+2d
a9=a1+8d
{a1+2d=7
{a1+8d=18
6d=11
d=11/6=1 5/6
a1=7-2*(11/6)=21/3-11/3=10/3
a6=a1+5d=10/3+5*(11/6)=75/6=25/2=12,5
a3=7
a9=18
a3=a1+2d
a9=a1+8d
{a1+2d=7
{a1+8d=18
6d=11
d=11/6=1 5/6
a1=7-2*(11/6)=21/3-11/3=10/3
a6=a1+5d=10/3+5*(11/6)=75/6=25/2=12,5