The monthly cost (in dollars) of water use is a linear function of the amount of water used in hundreds of cubic feet, HCF). The cost for using 16 HCF of water is
$31.19, and the cost for using 39 HCF is $66.84. What is the cost for using 19 HCF of water?

the answer is y < -3 or   y > 3

-3y > 7 + 2

-3 y > 9

y < -3

hello from mrbilldoesmath!

answer:

(7 x + 6 y) (49 x^2 - 42 x y + 36 y^2)

discussion:

the trick is to recognize that 343 = 7^3 and 216 = 6^3 and, in general,

a^3 + b^3 =   (a + b) (a^2 - a b + b^2).

in our case

343x^3+216y^3   =   (7x)^3 + (6y)^3

so take a = 7x and b = 6y in the above equation to get the answer.

you,

mrb