how would you write an equation or inequality that could be used for analysis, prediction example gas prices?

can some one explain to me how to write this

i was going to do mine on diet
how my sister last pounds on her diet she weight 410 lbs to begin with lost 1 year 110 lbs 2 nd year she 110 lbs
she weighs 190 lbs now

ok, now you have given me something to work with.
I see 3 ordered pairs
(0,410)
(1,300)
(2,190) the first no being years, the second number being weight.

notice if you take the slope between any two points, you get the same, slope = -110, so it must be a linear function

it is easy to see that the function is

weight = -110n + 400

there is an obvious restriction on the value of n, she can't keep losing weight like that for long
e.g. if n=5 she would have disappeared.

use a similar method for other analysis problems if you are given data that follow a mathematical pattern.