Normal distribution is calculated by addition, subtraction, multiplication and division:

Any linear combination of two normal distributions still obeys the normal distribution (which can be proved by finding the distribution of the functions of two normal distributions), and this conclusion can be extended to n normal distributions. So we only need the expected variance of X-3Y to know what normal distribution we obey. E(X-3Y)=E(X)-3E(Y)=-2，D(X-3Y)=D(X)+9D(Y)=29，X-3Y~N(-2，29)

Extended data:

Common causes of normal distribution:

Usually, there are many factors that affect a thing, such as everyone's height, which are influenced by many factors, such as:

1, height of parents;

2. Eating habits at home;

3. Do you exercise every day? What sports do you do every day?

Wait a minute.

Every factor, every day's behavior, is like flipping a coin. These factors will not have a positive impact on height, or will not have a negative impact on height, and eventually the overall height will be close to the normal distribution.