book Maths Puzzle Numerical Ability

7y+15=3x
15=3x+2y
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25x=91-16y
16y=64-16x
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4x-2y=-2
4x+3y=-12
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2x+y=-7
y=3x+3
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3x=-2y+10
x=2y=6
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book Other Question
I know it's simple, but I can not understand how to use internal notation and graphing, to answer:
X-X^2 <= 0

Thank you! is there a trick to finding a prime number