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This is Eric Hutchinson from the College of Southern Nevada. Thank you so much for watching!

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If we look to sign convention then the slope of the y-x graph i.e. ds/dt should be equal to -20mph as slope is tan(theta) and here theta the angle between slope and positive x-axis is more than 90 degrees in anticlockwise direction therefore slope will be negative as tangent of angle greater than 90 and less than 180 degrees is negative