Two perpendicular lines intersect at the origin. if the slope of the first line is 3, what is the equation of the second line? .

If slope of first line is 3 and is perpendicular to second line
then second line has a slope of -1/3

The slope of the line perpendicular to a line with a given slope is its negative reciprocal. Hence, the new slope is -1/3. The equation of the line given slope and point can be generally expressed as,
                                       y - y1 = m(x - x1)
Substituting,
                                      y - 0 = (-1/3)(x - 0)
The final equation would be,
                                      x + 3y = 0