[tex]b^{2}-15b-54=0. \newline
b^{2}-15b=54. \newline
b^{2}-15b+(\frac{15}{2})^{2}=54+(\frac{15}{2})^{2}. \newline
(b-\frac{15}{2})^{2}=\frac{441}{4}. \newline
b-\frac{15}{2}=\pm\frac{21}{2}. \newline
b=\pm\frac{21}{2}+\frac{15}{2}. \newline
b_1 = -3, b_2 = 18.[/tex]
