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1. 8 is the same as 2^3, so following rules of exponentials 8^x would equal 2^(3x)
2. Rearrange the eqn given in terms of y, i.e y=3x-12
3. Substitute this answer in place of y in the second equation, as everything is now in terms of x the eqn is now solveable. 2^(3x) / 2(3x-12) is the current form.
4. Use the rules of exponentials again, that 2^(a+b) = (2^a)*(2^b)
5. So now it's 2^(3x) / (2^(3x)*2^(-12)) and if you write it out you'll see that the 2^(3x)'s on the top and bottom will cancel
6. This leaves you with 1/(2^(-12)) which of course is identical to 2^12
Hope this makes sense, i'll try to clarify if not or if i messed up somewhere