I'm assuming that this: $\frac79$ and $\frac45$, means: 7/9 and 4/5 ??
If so, the nth term =First term + [common difference * Term number - 1]
The 13th term =7/9 + [D * (13 - 1)]
4/5 =7/9 + 12D, solve for D
D = 1 / 540
7th term =7/9 + 1 /540 * 6
7th term = 71 / 90
