The simplified answer is 4/5
$13 (?) off the original price of 240 is 229. She will have to pay $229.00

Actually Welcome to the concept of calculations of bills.
The bill calculation goes as equation,
y = $250 + n*($10)
here, n = number of lines,
1.) For n = 2 , Y = cost = 250 + 20 = $270
2.) for n = 4 , Y = $290
3.) for n = 6 , Y = $310
4.) for n = x, Y = $250 + x*($10)
for lastly 12 members we get as,
Y = 250 + (12)*(10) = 250+120 = $370
The answer would be 10.19m. If you want to round it of to a less precise measurement it would be 10 m. That is rounding of the decimal value to the ones place. the rule of rounding of is form 6 t0 9 it would add 1 to the nearest place value next to it and 0 - 5 will not.
To graph the equation, plot a few points then connect then with a line.
f(x) = 440 - 55x
Plug in numbers for x to get the y value.
f(0) = 440
f(1) = 385
f(2) = 330
f(3) = 275
f(4) = 220
f(5) = 165
So when you graph these points, the y intercept is y = 440 because the graph crosses the y axis at (0, 440).
The x intercept is where the graph crosses the x axis. This happens when y = 0.
0 = 440 - 55x. Solve for x.
55x = 440
x = 8
So the x intercept is x = 8