Data:
Quiz taken: 16
Quiz each week: 1
You can express this as a equation:
Q= Total of quizes taken
W= weeks
As each week the number of quizes Q increase in 1 the change is k=1, and Isabela has already taken 16 quizzes. The number of quizzes can be represented as:

Q0 is the number of quizzes Isabela has already taken.

Then, to have a total of 26 quizzes Isabela have to attend:
Q=26
W=?

You can find the number of weeks by clearing of the equation above the W:
Substract in both sides of the equation 16:


Then, Isabela have to attend 10weeks this quarter to have a total of 26 quizzes