Recall, the singly$-$reinforced beam capacity equation is shown$-$Please refer to NCEES FE$/$Civil Reference Handbook, Beams$-$Flexure:

$M_n=\frac{M_u}{}=A_s{F}_y(d-\frac{a}{2})=A_s{F}_y(d-\frac{1}{2}**\frac{A_s{F}_y}{0\mathrm{.}85*f_c^{\text{'}}b})$

$A_s=80\mathrm{.}79i{n}^2=6\mathrm{.}32i{n}^2$

$d=30in-2\mathrm{.}5in-\frac{2in}{2}=26\mathrm{.}5in$

$\frac{648ft-\text{k}\text{i}\text{p}**12i\frac{n}{f}t}{0\mathrm{.}9}=8640k-in$

$8640k-in=6\mathrm{.}32i{n}^2**60$ksi$**(26\mathrm{.}5in-\frac{1}{2}**\frac{6\mathrm{.}32i{n}^2*60\text{k}\text{s}\text{i}}{0\mathrm{.}85*4\text{k}\text{s}\text{i}*b})$

$b=14\mathrm{.}98in$