[tex]y=kx+b\\\\A(5;0):\; \; 0=5k+b\\\\B(-2;21):\; \; 21=-2k+b\\\\\left\{\begin{array}{c}5k+b=0\\-2k+b=21\end{array}\right\; \ominus \; \left\{\begin{array}{l}7k=-21\\b=2k+21\end{array}\right\; \; \left\{\begin{array}{l}k=-3\\b=15\end{array}\right\\\\\\\boxed {\; y=-3x+15\; }[/tex]