AOPS question that I don't understand. Please give the formula for this question. I have a contest tomorrow!

Me what is the value of x? showing work would be appreciated - ! : )

[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?

Suppose that a households monthly water bill (in dollars) is a linear function of the amount of water the household uses (in hundreds of cubic feet, hcf). when graphed, the function gives a line with slope of 1.65. if the monthly cost for 13 hcf is $46.10 what is the monthly cost for 19 hcf?