Question

Help please, I could not figure this problem because the answer was -108 but I only solved it to -244. The problem is “Find coefficient of x in expansion of (x-3)^4”

Answer 1

You already know the answer is -108

Explanation:

There is probably a formula for finding the coefficient of a variable in an expansion. But (x-3) is easy to square, and that gives us

x^2 - 6x +9

Now to square that quadratic trinomial I used an area model. You can get the entire expansion. But we are just only interested in the x terms. See image. They are in yellow on the image. You can see we get -54x two times. So

-54x + -54x is the -108x term you are interested in.