Given the sequence
[tex]a_n = 2+(n-1)(-2)=2-2(n-1)=2-2n+2= -2n+4[/tex]
You can compute the k-th term by evaluating the generic term at n=k. So, the 15th, 18th and 31st terms are
[tex]a_{15}=-2(15)+4=-30+4=-26[/tex]
[tex]a_{18}=-2(18)+4=-36+4=-32[/tex]
[tex]a_{31}=-2(31)+4=-62+4=-58[/tex]
