He should get back 44.68 not including any tax
Multiply, then you get 2a - 10 = -2. Then isolate the variable by adding 10 on both sides. you get 2a = 8. divide both by 2 and you get a = 4
It is 12 because:
18/6=x/4
Multiply 4 on either side
72/6=x
Simplify
12=x
Hope this helps!!