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GCSE Maths - Functions, Year 11 (Help!)

Help, I'm doing practice questions and I'm not sure what to do here..

For all values of x, f(x)= 3x + k where k is a constant .
Given that ff(3) = 7 , find value of k.
I'm sure if this question said f(3) = 7 you'd be able to get it and it's only a step more than doing that really.

All it is is, you are getting your answer from f(3), by substituting 3 in as x, and then using this answer to put in as your new 'x' value,
for your function 5(x), which you can equate to 7 and solve for k.

Here's how to layout your workings:

f(x) = 3x + k

f(3(3) +k) = 7 All this step is doing is finding the value for f(3) and then showing that you are putting your answer back into your function as a new value for 'x'

3( 9 + k) + k =7 This is what you get when you substitute your old answer, 9 + k, for your new value of x for the 3x + k

From here I believe you're more than capable of rearranging and solving for k.
(edited 6 years ago)
ff(x) = 3(3x + k) + k = 9x + 4k
ff(3) = 27 + 4k = 7
4k = -20
k = -5

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