Hi!
To answer this, first we have to find how much 1 notebook costs. We can do this by dividing:
Cost/amount = ?
Or in this case
5.25/5 = 1.5
Now we have to find how much 3 notebooks cost. We can do this by multiplying:
Cost of one x amount = ?
Or in this case
1.5 x 3 = 3.15
The answer is 3.15
Hope this helps! :)