Question

show that three points (4,2) (7,5) and(9,7) lie on a straight line

Answer 1

Slope of line joining (4,2) and (7,5) :  (5-2)/(7-4) = 1
slope of line joining :(7,5) and (9,7) =  2/2 = 1

so the given 3 points lie on a straight line.