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
1) 2000 because it is the closest number to 4000
2) 700*23/100 = 161
3) (-6+4)/(6-1) = -2/ I have bean asked to consider in detail the question x2 =-5x-6
I know how to solve the equation, however I have a feeling I could be asked why you put it equal to zero? Could you please tell me the reason along with any answers to any other questions you think I may be asked.?It's for an interview for teacher training. Thankyou