book Maths Puzzle Numerical Ability

how do you solve this problem:
y=2x-8
x=y+4

this problem is making me go mad!!!!
HELP ME PLEASE!!!!!!!!!!

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book Other Question

A box with a square base no top is to be made from a square piece of tin by cutting out a 3cm square from each corner and folding up the sides. If the box is to hold 48cm3, what size piece of tin should be used. Please provide the lowest possible factors for the following:

21z^3+28z