Posted by Jess on Tuesday, April 28, 2009 at 7:11am.
Hello, i'm currently doing functions and I was fine until I got to multiplication. I really don't know what i'm doing, and would really appreciate a step by step on a few questions.
f(x) = 3x + 9
g(x) = 8x + 7
Find g(f(x))
I got 24x + 16.
Another :
f(x) = 3x + 9
g(x) = 8x + 7
Find f(g(x))
Thanks in advance.

Math  Algebra II  Reiny, Tuesday, April 28, 2009 at 7:50am
no, one term was right, but your constant is wrong.
do it this way :
f(x) = 3x + 9
g(x) = 8x + 7
then g(f(x))
= g(3x+9)
= 8(3x+9) + 7
= 24x + 72 + 7
= 24x + 79
and f(g(x))
= f(8x+7)
= 3(8x+7) + 9
= 24x  21 + 9
= 24x  12 
Math  Algebra II  Jess, Tuesday, April 28, 2009 at 8:15am
Thank you so much! I've been sitting here for hours. Only 4 to go. Thanks :)

Math  Algebra II  Jess, Tuesday, April 28, 2009 at 8:40am
2 down.
I'm having trouble here
f(x) = 4x + 2
h(x) = 7x  5
Find f(h(x))
I tried doing it how it was done above, but i'm not getting an answer listed.
= f(7x  5)
= 4x(7x5)+ 2
= 28x^220+2
= 28x^218
Where am I going wrong : ?
ALSO 
f(x) = 4x3
g(x) = x22
Find f(3a4) 
Math  Algebra II  Reiny, Tuesday, April 28, 2009 at 8:58am
from
= f(7x  5) to
= 4x(7x5)+ 2
you are replacing the x, so don't leave the x there.
should be
= 4(7x5)+ 2
= 28x  20 + 2
= 28x  18
for
f(x) = 4x3
g(x) = x22
Find f(3a4)
I will assume g(x) = x^2  2
then f(3a4)
= 4(3a4)  3
= 12a  16  3
= 12a  19
the x in f(x) was merely replaced by 3a4, so you have to replace it everywhere that you see an x.