Answer
$72$ inches;
$ a_n=-5+7n$
Work Step by Step
The general formula for the sum of the nth term of an arithmetic series is given by $S_n= \dfrac{n}{2}(a_1+a_n)$
We know that $a_n=a_1+(n-1)d$ ....(1)
Equation (1) gives: $a_n=9+(n-1)\times 7$
This gives: $a_n=2+7n$
But the height at the top of the 10th stair is $2+7 \times 10=72$ inches
Thus, the height of the bottom of the n-th stair is: $ a_n=2+7n-7=-5+7n$
