u are trying to find 290 which is between 240 and 300
so u are trying to find something that is higher than 240 and lower than 300.. which is 290
its called interpolation
see brother for example
at FL 240 the wind is 114kt and at FL300 wind is 110 Kt
Look at it , as u are going up the wind is decreasing .. at FL240 it is 114 and at 300 it is 110 So FL 290 is between them , meaning the value should be less than 114 and greater than 110 , so that is why u subtract(-) from 114 , as u go up the wind is decreasing so it wil be -2 ,
So when u look at the problem u can see when to subtract and when to add ,
U could also solve it in another way
At FL300 wind is 110 and at FL 240 it is 114
Here u can look at it like this , As u go Down the wind Increases ,
so when u interpolate between 114 and 110 u get 2
This two u will add to 110 and u will get 112 , It depends where u are starting from , u will get the same answer,
Look at it like this , It cannot be 114 + 2 coz then it will not make sense , ur answe should be between 114 and 110 , so it cannot be 116 , so obvioulsy it has to be 112 coz 112 is between 110 and 114 just like FL 290 is between FL 240 and FL 300