How to solve -3x-6y=3 and 2x+7y=-17 using the elimation method Please show work but no link

What aspects of lines have to be diffrent

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Scores on the cognitive scale of the bayley-scales of infant and toddler development range from 1 to 19, with a standard deviation of 3. calculate the margin of error in estimating the true mean cognitive scale score, with 99% confidence, if there are 12 infants and toddlers in a sample.