I dont know what you want to know but I'll answer all of them.
for a it would be
[tex]3 + 2x[/tex]
for c you would exchange the variable x with 5 to find out the price they payed
for d, you would do the inverse operation and first subtract 19 by 3 and get 16. than divide 16 by 2 and you will get x = 8
