The slope of that line is 1/3 using rise over run method. So you create the equation for the line shown y= 1/3x-1. Now to find the perpendicular line to the line showed in the image you find the negative reciprocal of the slope which means to flip to the opposite sign and swap the numerator and denominators. This makes the slope from -3 to 1/3. So the slope of the line perpendicular to the line shown is 1/3. I also put it in slope-intercept form just to help you out.
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