$\begin{align}  & {{T}_{3}}=-9 \\ & {{T}_{7}}=-29 \\ & {{T}_{n}}=a+(n-1)d \\ & {{T}_{3}}=a+(3-1)d=-9 \\ & a+2d=-9---(i) \\ & {{T}_{7}}=a+(7-1)d=-29 \\ & a+6d=-29---(ii) \\ & \text{subtract (2) from (1)} \\ & -4d=20 \\ & \text{   }d=-5 \\ & \text{substitute }-5\text{ for }d\text{ in equation (i)} \\ & a+2(-5)=-9 \\ & a=1 \\ & {{T}_{10}}=1+(10-1)(-5)=1-45=-44 \\\end{align}$