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