Подставим координаты точек в уравнение и решим систему уравнений
\left \{ {{a=5-b} \atop {-1=-2a+b}} \right.<=> \left \{ {{a=5-b} \atop {-1=-2(5-b)+b}} \right.<=>\\\ \\\ <=> \left \{ {{a=5-b} \atop {-1=-10+2b+b}} \right.<=> \left \{ {{a=5-b} \atop {3b=9}} \right.<=> \left \{ {{a=5-b} \atop {b=3}} \right. <=> \\\ \\\ <=> \left \{ {{a=5-3} \atop {b=3}} \right.<=> \left \{ {{a=2} \atop {b=3}} \right." alt="\left \{ {{5=a+b} \atop {-1=-2a+b}} \right. <=> \left \{ {{a=5-b} \atop {-1=-2a+b}} \right.<=> \left \{ {{a=5-b} \atop {-1=-2(5-b)+b}} \right.<=>\\\ \\\ <=> \left \{ {{a=5-b} \atop {-1=-10+2b+b}} \right.<=> \left \{ {{a=5-b} \atop {3b=9}} \right.<=> \left \{ {{a=5-b} \atop {b=3}} \right. <=> \\\ \\\ <=> \left \{ {{a=5-3} \atop {b=3}} \right.<=> \left \{ {{a=2} \atop {b=3}} \right." align="absmiddle" class="latex-formula">
уравнение прямой имеет вид y=2x+3