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Ответ:

(3; -2)

[tex]\left \{ {{a*3+3*(-2)=15} \atop {2*3-2b=8}} \right. \\\left \{ {{3a-6=15} \atop {6-2b=8}} \right. \\\left \{ {{3a=21} \atop {2b=-2}} \right. \\a = 7\\b = -1[/tex]

[tex]\left \{ {{ax+3y=15} \atop {2x+by=8}} \right. \\\left \{ {{3a-6=15} \atop {6-2b=8}} \right. \\\left \{ {{3a=21} \atop {-2b=2}} \right. \\\left \{ {{a=7} \atop {b=-1}} \right.[/tex]

Вместо х подставляешь 3, а вместо у - -2b.