Let x be the number of cases of paper towels and y be the number of cases of soap. The system of linear equation that would best represent the given is,
x + y = 10
12x + 15y = 129
The values of x and y are 7 and 3.
Sarah both 7 cases of paper towel.
It is not possible for Sarah to buy exactly 20 bottles because first of all the cases contains only 12 bottles and 20 is not divisible by 12.
The final equation is:

Further explanation:
The general form of equation of a line in slope-intercept form is:

Here m is the slope
Given
m=1
(x,y) = (-7,-4)

To find the value of b, we have to put the point in the equation

The final equation is:

Keywords: Slope-intercept form, equation of line
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I do not know this one but the first garden is 24 and the second one is 576.