In advance sorry for the very collumn layout but I tihnk it makes the question a lot clearer.

un+1 = aun + b

u1 = 5,
u2 = 9.5,
u3 = 20.75

so

9.5= a(5) +b
20.75= a(9.5) + b

now I am not sure what to do from here one, should you combine both the equations or do a simultanious equation?

Combining

30.25 = 2a(47.5) +2b

x/2

15.125 = a(47.5) + b

15.125 - b = a(47.5)

(15.125 -b)/47.5 = a

I'm not sure if this is the right route since I have a and b on either sidebut I'm lost from here, any help would be appreciated:)

You actually need to solve them simultaneously.

I don't see any point in combining them this way, though that can work for some equations. The general way we call is solving simultaneously by elimination. If you want to 'combine' the equations, you'll be better off subtracting one from the other.

Can you try it now and post what you get? :)

9.5= a(5) +b
20.75= a(9.5) + b

11.25=a(4.5)+ b

is the rest guess work because I still have two variables on a side

Come on!

Code:

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20.75 = 9.5a + 1b  -
9.50 = 5.0a + 1b
11.25 = 4.5a + 0b

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:)

ah thank you so much =] what's probably quite scary is that I got an A yesterday in a practice paper, its these silly things like this that trip me on my face. Thanks

No worries, that sometimes happens, better double check for those kinds of mistakes. These ones are quite easy to spot because you have one b in each equation and by elimination, it is only natural that none should remain. If you are more familiar with other methods such as substitution, use them as a check.

Good luck! :)