Question 81300
You can't; you have two many variables.

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Let's say you want to vary the price of gas but want  to
purchase no more than 12 gallons of gas per week to help
conserve fuel.
Then Cost = 12p where p is the price per gallon for gas.
Then you have a graph that looks like:
{{{graph(400,300,-5,5,-5,100,12x)}}}
On this you can read the cost of your gas as it changes
from $0 per gallon to $5 per gallon.
Cheers,
Stan H.