Get opposite of number if middle is not zero

I need a function that will return the opposite of a number, but i also need to be able to specify the 'center'.

eg. Center = 0. If the number was 10, it would return -10.

I need this: Center = 100. If the number was 10, it woul开发者_运维技巧d return 190.

Any idea how it could be done?

Thanks.

-(int) returnOppositeOf:(int)myNumber aroundCenter:(int)center
{

return ( (center*2) - myNumber);

}

Opposite = 2 * Center - X

opp = center - (x - center)