exam Maths Puzzle Numerical Ability

solve by completing the square. 4x²+2x-3=0

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exam Other Question

I'm lost on this one.
A data set of size 19 has a correlation coefficient of r= -0.432. Test the significance of r at the 5% level, and at the 1% level.
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Please solve this problem: (3x-2y)squared

Answer sheet says it 9x squared+4y squared-12xy.

I understand the 9x squared + 4y squared but not the -12xy.

Thanks,

S