Воспользуемся основным тригонометрическим тождеством:
\({\sin ^2}A + {\cos ^2}A = 1\,\,\,\,\,\,\, \Leftrightarrow \,\,\,\,\,\,{\left( {\frac{7}{{25}}} \right)^2} + {\cos ^2}A = 1\,\,\,\,\,\,\, \Leftrightarrow \,\,\,\,\,\,\,\cos A = \sqrt {\frac{{576}}{{625}}} = \frac{{24}}{{25}}.\)
По определению косинуса из треугольника ABC:
\(\cos A = \frac{{AC}}{{AB}}\,\,\,\,\,\,\,\, \Leftrightarrow \,\,\,\,\,\,\,\,\frac{{24}}{{25}} = \frac{{AC}}{5}\,\,\,\,\,\,\,\, \Leftrightarrow \,\,\,\,\,\,\,\,AC = \frac{{24}}{5} = 4,8\).
Ответ: 4,8.