Let x be the number of weeks that it will take Andy to reach his goal.
We have been given that Andy currently runs a total of 12 miles per week. He plans to increase that number by 1.5 miles, so number of miles increased in x weeks will be 1.5x and total number of miles ran in x weeks will be 1.5x+12.
Since Andy wants to reach a total of 26 miles per week, so we will equate the total number of miles ran in x weeks with 26.
Therefore, the equation can be used to determine x, the number of weeks that it will take Andy to reach his goal.