Answer: 700x+1700
This represents the total amount spent over the x weeks that have gone by.
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Step-by-step explanation:
x = number of weeks
700x = amount spent over those x weeks ($700 per week)
700x+1700 = total amount spent when adding on the $1700 tv.
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An example:
Let's say x = 10 weeks have elapsed. He spends $700 a week, so 700x = 700*10 = 7000 dollars have been spent so far if we ignore the tv. Then add on the $1700 tv cost to get 7000+1700 = 8700 dollars total spent.
Then note how 700x+1700 = 700*10+1700 = 8700 matches what we got earlier.
I recommend trying out other values of x to get a better idea of what's going on.