Hello, Andy!

You didn't give us the rule for filling the boxes.

I will assume that it is an "additive" sequence . . .

Code:

* - - - *
| 42 |
* - + - * - + - *
| | |
* - + - * - + - * - + - *
| | | |
* - + - * - + - * - + - * - + - *
| 4 | T | 3 | 5 |
* - - - * - - - * - - - * - - - *

Assuming that each number is the sum of the two numbers below it,

. . we have this pyramid. Code:

* - - - *
| 42 |
* - + - * - + - *
| 2T+7 | T+11 |
* - + - * - + - * - + - *
| T+4 | T+3 | 8 |
* - + - * - + - * - + - * - + - *
| 4 | T | 3 | 5 |
* - - - * - - - * - - - * - - - *

The final "step" would be: .$\displaystyle (2T+7) + (T + 11) \:=\:42$

And we get: .$\displaystyle 3T + 18 \:=\:42 \quad\Rightarrow\quad 3T \:=\:24 \quad\Rightarrow\quad\boxed{ T \:=\:8}$