X=cost of one box of popcorn.
we can suggest this equation:
3x+1.75=9.25
3x=9.25-1.75
3x=7.5
x=7.5/3
x=2.5
Answer: the equation to find the cost of one box of popcorn is 3x+1.75=9.25, and the cost of one box of popcorn is $2.5.
No, when you have a zero in that space you don't necessarily need an addendum unless it helps you.
Since you have y alone on one side of the equation......
Use substitution and plug in 1/2x-4 for y in the other equation.

Distribute

Combine like terms

Add 8 to both sides

Divide by 4
and 
Plug this value in for x

Simplify

Multiply everything by 8 and simplify

Divide.....

Your answers are:

This is the answer

if substitute 1 and 2 and 3 as x then the points will be :

and that's the picture
good luck