7people shake hands. give a formula to represent the number of handshakes if n is the number of people

One line passes through (-7,-4) and (5,4) . another line passes through the point (-4,6) and (6,-9)

Aperpendicular bisector, cd is drawn through point con ab if the coordinates of point a are (-3, 2) and the coordinates of point b are (7,6), the x-intercept of cd is point lies on cd.

Select the lengths in centimeters that would form a right triangle