\[y = kx + b;\ \ C(0;15);\ \ D( - 5;0).\]
\[C(0;15):\]
\[15 = 0k + b\]
\[b = 15.\]
\[D( - 5;0);b = 15:\]
\[0 = - 5k + 15\]
\[- 5k = - 15\]
\[k = 3.\]
\[Ответ:k = 3;\ \ b = 15.\]