Объяснение:

[tex]\displaystyle\\2x^2+x-a=0\ \ \ \ \ \ x_1=3\ \ \ \ \ \ x_2=?\\\\2x^2+x-a=0\ |:2\\\\x^2+0,5x-0,5a=0\\\\a=1\ \ \ \ b=0,5\ \ \ \ c=-0,5a\\\\ \ \ \ \ \ \left \{ {{x_1+x_2=-b} \atop {x_1*x_1=c}} \right. \ \ \ \ \left \{ {{3+x_2=-0,5} \atop {3*x_2=-0,5a}} \right. \ \ \ \ \left \{ {{x_2=-3,5} \atop {3*(-3,5)=-0,5a}} \right.\ \ \ \ \left \{ {{x_2=-3,5} \atop {-10,5=-0,5a\ |*(-2)}} \right.\\\\\\\left \{ {{x_2=-3,5} \atop {a=21}} \right. .[/tex]

Ответ: а=21.