First we need to find the slope of line k. Using the slope formula, we fill in accordingly: . The slope is -3/2. The slope of a line that is perpendicular to line k will have the opposite sign (so, positive) and will be the reciprocal of the line k's slope (so, 2/3). You could also say, more "mathematically legal", that the product of the slopes of perpendicular lines is -1, but it's easier to just say that they are opposite reciprocals. The slope you're looking for is 2/3. Choice 3 above.
Division by zero is an operation for which you cannot find an answer, so it is disallowed. You can understand why if you think about how division and multiplication are related. But no value would work for x because 0 times any number is 0. So division by zero doesn't work.