The coordinates of the midpoints are the mean of the coordinates of the two points.
Point P has coordinates [tex](0,0)[/tex], because it is the origin of the coordinate system.
So, if we let T be the midpoint of PR, we have
[tex]T = \left(\dfrac{0+(2a+2b)}{2},\dfrac{0+2c}{2}\right) = (a+b, c)[/tex]