First find the slope of A.
Slope=(y2-y1)/(x2-x1)=(4-0)/(0--7)=4/7
For a line to be perpendicular to line A, it must have a slope that is the negative reciprocal of the slope of A. Mathematically:
m1*m2=-1 and in this case the perpendicular line will have a slope of:
4m/7=-1
4m=-7
m=-7/4
So the slope of the line perpendicular to A is -7/4
