I think 70 because playing on a varsity, i think, means to play both sports. So I added 10 to the number of students that play basket ball, which is also equal to the number of students that like tennis.
you need to find lines whose slope is the negative reciprocal of the slope from the line you are solving for. in this case, all lines with the slope -1/2 are Perpendicular to a line with the slope of 2