Linear Combinations (Probability) question

I would like to start by thanking mrfantastic for solving BOTH my prob. questions (Bow). Thank you!!

Anywayz, heres my question:

You know the rule for Linear Combinations where the Variance of say, 2X + 3Y is something like Var(2X+3Y) = 2^2Var(X) + 3^2Var(Y)? But how come on some questions I came across you don't square the co-efficient (2 and 3) ?

(i.e Var (2X+3Y) = 2Var(X) + 3Var(Y) )

In other words, what conditions must be met in order to square it or not? (please give some examples)

Thanks in advance! =D