Hey there! I'm happy to help!
Our question tells us that the number of packages is p and that the number of binders is b.
Each package is $3, so for every p we have, we will have 3 more dollars. So we have 3×p, which is written as 3p.
The same goes for binders. For every binder, we have 4 more dollars, so we have 4×b which is just written as 4b.
An expression is just a number sentence without an equal sign, so something like 1+1 or 7x². The variables are just the letters or symbols that represent an unknown quantity (so 7x² is a variable expression).
Our total cost is going to be the cost of our packages and our binders, so we combine the 3p and 4b we previously came up with, so our expression is 3p+4b.
Now, we can plug in our amounts of each thing into the equation. We will insert our 3 packages in plae of the p and our 8 binders in place of the b to find our cost.
In algebra, you can also write multiplication as a number outside of a number in parentheses so 4(2)=8 for example.
3(3)+4(8)
9+32
41
So, the total cost is $41.
Have a wonderful day and keep on learning! :D