Sn=700=2n[2a+(n−1)d]...(i)
a6=7⇒a+5d=7...(ii)
S7=7⇒27(2a+6 d)=7
a+3d=1...(iii)
Solve (ii) and (iii)
$\begin{aligned}
& \frac{\mathrm{n}}{2}(-16+3 n-3)=700 \Rightarrow 3 n^2-19 n-1400=0 \
& (3 n+56)(\mathrm{n}-25)=0 \
& \therefore \mathrm{a}_{25}=\mathrm{a}+24 \mathrm{~d}=-8+24 \times 3 \
& =-8+72
\end{aligned}$
=64