$f_v = \dfrac{VQ}{Ib}$

Where:
$f_v = 80 \, \text{psi}$
$V = 100L$
$Q = 8(5)(2.5) - 6(4)(2) = 52 \, \text{in^3}$
$I = \frac{1}{12}(8)(10^3) - \frac{1}{12}(6)(8^3) = \frac{1232}{3} \, \text{in}^4$
$b = 8 - 6 = 2 \, \text{in}$
Thus,
$80 = \dfrac{100L(52)}{\frac{1232}{3}(2)}$
$L = 12.64 \, \text{ ft}$ answer