When two lines are perpendicular to each other, their slope are inverted. This means that if the slope of a line is "m", then the one that is perpendicular to it should be "-1/m". The slope-intercept equation of a line is given by:
[tex]y=mx+b[/tex]Where "m" is the slope. The line from this problem has the slope equal to 2, therefore the slope of the line that is perpendicular to it is:
[tex]m_2\text{ = }\frac{1}{m}=-\frac{1}{2}[/tex]