[tex]\displaystyle\bf\\\left \{ {{a_{2} -a_{6} =7,2} \atop {a_{4}=0,7 }} \right. \\\\\\\left \{ {{a_{1} +d-a_{1} -5d=7,2} \atop {a_{1}+3d=0,7 }} \right. \\\\\\\left \{ {{-4d=7,2} \atop {a_{1}+3d=0,7 }} \right.\\\\\\\left \{ {{d=-1,8} \atop {a_{1} =0,7-3d}} \right. \\\\\\\left \{ {{d=-1,8} \atop {a_{1} =0,7-3\cdot(-1,8)}} \right. \\\\\\\left \{ {{d=-1,8} \atop {a_{1} =0,7+5,4}} \right. \\\\\\\left \{ {{d=-1,8} \atop {a_{1} =6,1}} \right. \\\\\\Otvet \ : \ a_{1} =6,1[/tex]