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Grade 9 Maths Question

any mathematicians here know how to solve the following question :

f(x) = 2x + c
g(x) = cx + 4
fg(x) = 8x + d

c and d are constants
work out the value of d
So first lets do the fgx thing.
Put the g into the f equation:
2(cx+4)+c
=2cx+8+c

So 2cx+8+c is equal to 8x+d
c must equal 4, as 2*4=8

We can then sub this in, so it's: 2(4)x+8+(4)=8x+d
8x+8+4=8x+d
8x+12=8x+d
Therefore d=12

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