(2,5) I think I’m sorry if I’m wrong:/
The answer should be 11 since adding all the x’s up before 13 equals 11
Step 1: write the number as a product, helps visualizing common factors.

I've marked things being repeating, so, group them up

Now you have a common factor again

Extra, if you want to split the first one, you can see it as the difference of

and

which is further split into