3 In line 4 the last number is 7. What would be the last number in Line 7?

The sum of the first $n$ odd numbers is $n^2$. The $n$-th odd number is $2n-1$.
For Line 5, $n=5$.

The last number is $2(5) - 1 = 9$.
The sum is $5^2 = 25$.

The completed line is: $\boxed{\text{Line 5: } 1 + 3 + 5 + 7 + 9 = 25 = 5 \times 5}$

For Line 6, $n=6$.

The last number is $2(6) - 1 = 11$.
The sum is $6^2 = 36$.

The completed line is: $\boxed{\text{Line 6: } 1 + 3 + 5 + 7 + 9 + 11 = 36 = 6 \times 6}$

The last number in Line $n$ is the $n$-th odd number, which is given by $2n-1$.

For Line 7, $n=7$.
Last number $= 2(7) - 1 = 14 - 1 = 13$.
The last number in Line 7 would be $\boxed{13}$.

The sum of the numbers in Line $n$ is $n^2$.

For Line 12, $n=12$.
Sum $= 12^2 = 144$.
All the numbers in Line 12 would add up to $\boxed{144}$.