Question 728542
A line parallel to another line has the same slope. A line perpendicular to a line has the opposite sign reciprocal of the slope.
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So what is an opposite sign reciprocal, or reciprocal in general? A reciprocal is basically an upside down fraction. For example, the reciprocal of 3/5 is 5/3, or the reciprocal of 5 is 1/5. To make a reciprocal an opposite sign reciprocal, just change it to the opposite positive or negative. The opposite sign reciprocal of 3/5 would be -5/3.
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I'm pretty sure you mean (2,-1) should be (-2,-1), otherwise the equation 2x+3=y would be wrong. Point (0,3) can be found by putting 0 in as x and solving the equation 2x+3=y. We can now find our slope using the two points.
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The slope of the equation is 2. (If you don't know how to find slope, see the link below) The opposite sign reciprocal of 2 is -1/2, so the slope for your perpendicular line is -1/2. 
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How to find slope: http://www.mathwarehouse.com/algebra/linear_equation/slope-of-a-line.php