A line that is perpendicular to another one has a slope that is the opposite reciprocal of that slope. This means that it is the opposite sign and 1 over the slope. With this in mind, the opposite reciprocal of 1/3 is -3, so we are looking for lines with a slope of -3.
The first, third, and fifth equations have slopes of -3. This is seen by the fact that it (-3) is being multipled by x in the equation when y is the only variable on one side or isolated.
I hope this helps :D
