[tex]q=-\frac{2}{3} \\S_{4}=13\\ S_{4}=\frac{b1(q^{4}-1 )}{q-1}\\b1(q^{4}-1 )=S_{4} (q-1)\\b1=\frac{S_{4} (q-1)\\}{(q^{4}-1 )} =\frac{13(-\frac{2}{3}-1)}{((-\frac{2}{3} )^{4}-1 )} =27[/tex]