Since [tex]a_n[/tex] is an arithmetic sequence, there is some constant [tex]d[/tex] by which consecutive terms differ. This means
[tex]a_{16}=a_{15}+d[/tex]
[tex]a_{17}=a_{16}+d=a_{15}+2d[/tex]
[tex]a_{18}=a_{17}+d=a_{15}+3d[/tex]
[tex]a_{19}=a_{18}+d=a_{15}+4d[/tex]
[tex]168=136+4d\implies d=8[/tex]
By a similar token, we can express [tex]a_{15}[/tex] in terms of [tex]a_1[/tex]:
[tex]a_2=a_1+d[/tex]
[tex]a_3=a_2+d=a_1+2d[/tex]
[tex]\cdots[/tex]
[tex]a_{15}=a_{14}+d=a_1+14d[/tex]
[tex]136=a_1+14\cdot8\implies a_1=24[/tex]
