\[15 - 2x - x^{2} = 0\]
\[x^{2} + 2x - 15 = 0\]
\[x_{1} + x_{2} = - 2,\ \ \]
\[x_{1} \cdot x_{2} = - 15\]
\[\Longrightarrow x_{1} = - 5;\ \ x_{2} = 3.\]
\[15 - 2x - x^{2} =\]
\[= - (x + 5)(x - 3) =\]
\[= (3 - x)(x + 5).\]