please solve the math equation in php

hi all i have a problem not solved by me the math equation in php. following is behavior what i want.

i have 5 digits ( 1 , 2 , 3 , 4 , 5)

if it is 1 then provide a equation that return a digit -15
if it is 2 then provide a equation that return a digit -12
if it is 3 then provide a equation that return a digit -9
if it is 4 then provide a equation that return a digit -6
if it is 5 then provide a equation that return a digit -3

equation must same for all digits.
explained again as:

1 equation = -15
2 equation = -12
3 equation = -9
4 equation = -6
5 equation = -3

what will be equation?
i will be very thank full to try to solve my problem.

 

using a switch statement it can b solved.............

 

thanks jinGKing but i want to solve it by just one equation. i am working on a site that will be handle by this equation. not by lengthy code.

 

I think this is more a math problem than a PHP problem
Try to post in math forums. Come back when you have the equation and we'll do the php part...

 

thanks dujmovicv

 

Well, it seems a rather simple problem to me: y = 3x - 18

If x = 1, y = -15
If x = 2, y = -12
If x = 3, y = -9
If x = 4, y = -6
If x = 5, y = -3

Hence:

$result = (3 * $input) - 18

Code (markup):