book Maths Puzzle Numerical Ability

Can you please help me solve:
6x+28(4x-7)=9x

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book Other Question

if a = 5b
then 3/5(a) = ?

the answer is 3b but I don't understand the rationale behind it, please explain!? find the foci of the hyperbola with equation y�/144-x�/100=1