2 notebooks
Explanation:
the variables I'm gonna use are t for textbooks and n for notebooks
t = (n•2)+3 (two times the notebooks then add 3)
now we'll plug in the amount of notebooks we have so t=7
7=(n•2)+3
we can subtract 3 from each side that leaves us with
4=n•2
then divide by 2
2=n
Guillermo bought 2 notebooks