book Maths Puzzle Numerical Ability

a=b
a2=ab this is squared
a2-b2=ab-b2
(a+b)(a-b)=b(a-b)
a+b=b
b+b=b
2b=b
2=1 how is this possible what did i do wrong?

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book Other Question

Simplify: y to the 4th power times y to the -2nd power. What are the steps you take to solve? r+3s=9
3r+2s=13

Use substitution method to solve system.
If its dependant or inconsistant, so indicate