When must a system of linear equations be solved algebraically, not graphically

What is the best way to answer questions with a x expression in the question. for example 3x + 10x=?

Amanda went into the grocery business starting with five stores. after one year, she opened two more stores. after two years, she opened two more stores than the previous year. if amanda expands her grocery business following the same pattern, which of the following graphs represents the number of stores she will own over time?

How do you find the distance of each number from the mean