Formula to find the mid-point is
[tex] (\frac{x_{2}+x_{1}}{2} ,\frac{y_{2}+y_{1}}{2} ) [/tex].
We need to find the mid-point of HG where H(0,0) and G(2a,0). So, now we can plug in x1=0, y1=0, x2=2a and y2=0 in the above formula to get the answer. So, mid-point of HG is,
[tex] (\frac{0+2a}{2} ,\frac{0+0}{2} ) [/tex].
=[tex] (\frac{2a}{2} ,\frac{0}{2} ) [/tex]
=(a,0).
So, mid-point of HG is (a, 0).