How do i solve a literal equation. ex: mary, has to pay x kg meatballs, y kg potatoes and z litres juice

So first off, you need another bit of information. if we were looking at how much it would cost her. we would need to know the price of meatballs, potatoes and juice. from then, it is simple

we have x times by the cost of meatballs per kilo, y times by the cost of potatoes per kilo, and z times by the cost of juice per litre. 

so, if meatballs were $10 a kilo, potatoes $5 and juice $1. it would be

10*x + 5*y + 1*z

that's the total cost