Find the midpoint of the line segment joining the points P1 and P2.
I also have a picture of the problem.

Formula for midpoint between two points is M(x,y)
x=(x1+x2)/2 and y=(y1+y2)/2
In our case (x1,y1)=(m,b) and (x2,y2)=(0,0)
x=(m+0)/2=m/2 and y=(b+0)/2=b/2 M(m/2,b/2)
Good luck!!!