[tex]\displaystyle\bf\\a_{18} =5\\\\a_{19} =3\\\\\\a_{19} =a_{18} +d\\\\d=a_{19} -a _{18} =3-5=-2\\\\a_{18} =a_{1} +17d\\\\a_{1} =a_{18} -17d=5-17\cdot(-2)=5+34=39\\\\Otvet \ : \ a_{1} =39[/tex]