$\begin{align}  & {{a}_{1}}=-8,n=14,{{S}_{n}}=70 \\  & {{S}_{n}}=\frac{n}{2}({{a}_{1}}+{{a}_{n}})\Rightarrow {{S}_{14}}=\frac{14}{2}(-8+{{a}_{14}})=70 \\ \end{align}$ $\Rightarrow 7(-8+{{a}_{14}})=70\xrightarrow{taghsim\,tarafeyn\,bar\,''7''}-8+{{a}_{14}}=10$ $\begin{align}  & \Rightarrow {{a}_{14}}=18 \\  & {{a}_{14}}={{a}_{1}}+13d\Rightarrow 18=-8+13d\Rightarrow d=2 \\  & \Rightarrow {{a}_{13}}=18-2=16 \\ \end{align}$