algebra question, please help, thanks in advance

Algebra
Tutor: None Selected Time limit: 1 Day

Let f(x)=x^2-13 and g(x)=9-x      

Perform the composition or operation idicated.

(fg)(-4)=

Please explain how this works 

Sep 23rd, 2015

Thank you for the opportunity to help you with your question!

There are two ways to do this:

First you can take f(-4) * g(-4) = [(-4)^2 -13] * [9-(-4)] = [16-13] * [13] = 3 * 13 = 39

Second, you can multiply the two functions:  (x^2 -13) (9-x) = 9x^2 - 117 - x^3 + 13x

= -x^3 + 9x^2 + 13x - 117

Then, we plug (-4) in for "x":  [-(-4)^3 + 9(-4)^2 + 13(-4) - 117] = 64 + 144 - 52 - 117 = 39

Please let me know if you need any clarification. I'm always happy to answer your questions.
Sep 23rd, 2015

Thank you so much, I just get so confused with how to do it.

Sep 23rd, 2015

Did you understand my explanation?

Sep 23rd, 2015

yes, thank you

Sep 23rd, 2015

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