Book Maths Puzzle Algebra

Please explain how the simplifying works to get from:
D/3 = 4 + D/9,
TO:
3D = 36 + D
Also, where could I find more about this type of problem?

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Book Other Question

how do you convert y=-1/2x+5 to standard form? 5q^2+12q-9 divided by 5q^2-22q-15 ;then multiply by 20q^2=12q divided by 9-q^2
I am at a loss to factor this in order to come to a solution. Can you help?