Answer:
The best choices for adding a transition at the beginning of the final sentence are:
As a result
However
Therefore
Here's how these transitions could be used:
As a result: "As a result, Laura's weekly pay is equal to (156 + X) / 7."
However: "However, you don't know the exact value of X, so you can't find the exact value of Laura's weekly pay."
Therefore: "Therefore, you can't find the exact value of Laura's weekly pay."
The other choices, "For example" and "Similarly," do not fit as well in this context because they are used to introduce examples or to show that something is similar to something else, but they do not provide a logical connection between the previous information and the final sentence.