The key is to find the first term a(1) and the difference d.
in an arithmetic sequence, the nth term is the first term +(n-1)d the firs three terms: a(1), a(1)+d, a(1)+2d the next three terms: a(1)+3d, a(1)+4d, a(1)+5d, a(1) + a(1)+d +a(1)+2d=108 a(1)+3d + a(1)+4d + a(1)+5d=183
subtract the first equation from the second equation: 9d=75, d=75/9=25/3 Plug d=25/3 in the first equation to find a(1): a(1)=83/3
the 11th term is: a(1)+(25/3)(11-1)=83/3 +250/3=111
