SOLUTION: I am trying to understand the distributive property III. When I do the problem, I add all the numbers together to get one number. My mom says that is wrong and I don't understand

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Question 163424This question is from textbook Pre-Algebra
: I am trying to understand the distributive property III. When I do the problem, I add all the numbers together to get one number. My mom says that is wrong and I don't understand why. Two problems I'd like help are:
-7(-3n + 2) my answer is 7n
7(2m + t) my answer is 21
Please help. Thank you for your time. This question is from textbook Pre-Algebra

Answer by Earlsdon(6294) About Me  (Show Source):
You can put this solution on YOUR website!
Listen to your mum!
-7%2A%28-3n%2B2%29 Apply the distributive property.
-7%2A%28-3n%29%2B%28-7%29%282%29 Simplify.
21%2An%2B%28-14%29 and this is becomes:
21%2An-14 Note! You cannot subtract the 14 from the 21n because they are different things. The 14 is a "constant" term while the 21n is a "variable" term. You cannot subtract these until you know what the value of the variable n is equal to. Trying to add/subtract these is probably one of the most-often committed errors in doing distributive property problems.
7%2A%282%2Am%2Bt%29 Apply the distributive property.
7%2A%282%2Am%29%2B7%28t%29 Simplify.
highlight%2814%2Am%2B7%2At%29 You cannot add the 14m to the 7t because, while they are both "variable" terms, the variables are not the same. It's sort of like trying to add apples and oranges, it just can't be done!