Or is your question just a basic Excel usage one?
Excel doesn't do "variables" as such. Instead, you put a value in a cell. And that cell is referenced later on.
Here's an example. Your "advertised" stall speed (Vs, or the bottom of the green arc) is in cell B2. You now want to know your stall speed under a certain load factor (which, itself, may be derived from the angle of bank.) Say, the load factor (G) is stored in cell B3 and the result should be in cell B4. Then B4 should have the contents
=B2*SQRT(B3)
It's that simple. If the contents of a cell start with an equal sign, it means it's a formula. And within that formula you can use the normal mathematical operators (+, -, *, / and so forth), functions such as SQRT, SUM and a whole library of others, numerical values and references to other cells.
There is a little known catch though. You can refer to a cell as B2, or as $B$2 (or as B$2 or $B2 - it depends on what you want to achieve). In the first case, if you copy and paste cell B4 (the formula) to cell C4, then the formula will automatically be changed into =C2*SQRT(C3). In the second case, $B will remain $B despite the downwards copy & paste. So if you are referring to cells that hold constants or one-time values in your formula, best to refer to them as $B$2 so if you copy & paste any formulas, the cell reference will remain unchanged. But if you copy and paste formulas that refer to values that are stored in tables themselves, it's better to refer to them as B2. The dollar sign sort-of "locks" the row or column, so it doesn't change when doing a cut&paste.