\[\boxed{\text{677\ (677).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[5x^{2} - 12x + c = 0,\ \ x_{1} = 3x_{2}\]
\[x^{2} - \frac{12}{5}x + \frac{c}{5} = 0\]
\[c = 15 \cdot \left( \frac{3}{5} \right)^{2} = 15 \cdot \frac{9}{25} =\]
\[= 3 \cdot 9\ :5 = \frac{27}{5} = 5,4.\]
\[Ответ:c = 5,4.\ \]