book Maths Puzzle Numerical Ability

How many diagonls can be drawn from each vertex of a decagon?

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book Other Question

I do not understand the steps to solving these problems.
#9: y less than or equal to 3x
What points do I start with.
#17: y > 1/3x + 1
Don't even know where to start with this 9a2+42a+49 what are the two numbers