Using the formula for the general term of an arithmetic sequence, we have:
an = a1 + (n-1)*d
an= 11 + (n-1)*2 (In this case the common difference 'd' is 2)
a4= 11 + (4-1)*2 (Replacing n=4 to find the fourth term)
a4= 11 + (3)*2 (Subtracting)
a4= 11 + 6 (Multiplying)
a4 = 17 (Adding)
The answer is 17