Answer
$a_1=10$
Work Step by Step
Sum formula:
$$S_n=\frac n2(a_1+a_n)$$
Use $n=16$:
$$S_{28}=\frac {28}2(a_1+a_{28})$$
Substitute $S_{28}=2926$ and $a_{28}=199$:
$$2926=\frac {28}2(a_1+199)$$
Solve for $a_1$:
$$2926=14(a_1+199)$$
$$a_1+199=209$$
$$a_1=10$$
