Let x and y represent the numbers of small and large cars washed, respectively.
We know that:
$4.50x + $8y = $420
But we don't know the relationship between x and y. We need more information (e.g., they washed twice as many small cars and large). Otherwise we have one equation and two unknowns. Since there are two unknowns, we can't determine a "slope" of the line. With all the water and soap, the most we could possibly say it will be a slippery slope.
