Answer
M = $(0.5a,0.5a)$
Work Step by Step
The coordinates (x, y) of the midpoint M are
$x = \frac{x_1+x_2}{2} = \frac{a+0}{2} = 0.5a$
$y = \frac{y_1+y_2}{2} = \frac{a+0}{2} = 0.5b$
That is, M = $(0.5a,0.5a)$
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