The formula to calculate the midpoint between two points is given by
[tex]M=(\frac{x1+x2}{2},\frac{y1+y2}{2})[/tex]we have the coordinates
A(0,0) and C(a,a)
substitute the given coordinates
[tex]\begin{gathered} M=(\frac{0+a}{2},\frac{0+a}{2}) \\ \\ M=(\frac{a}{2},\frac{a}{2}) \end{gathered}[/tex]