The foot of the perpendicular from the point  1,2 on the line passing through the points 5,4 and 7,4 is a,b then  a+b

Equation of the line joining two points 5,4 and 7,4 is y=4 , because y coordinates are same and equal to 4.Foot of the perpendicular from the point 1,2 on the line y=4 is 1,4 , which is   a,bHence  a+b=1+4=5