Answer:
(a,b)
Step-by-step explanation:
Using the Midpoint formula,
[tex]\left(\frac{x_2+x_1}{2},\:\:\frac{y_2+y_1}{2}\right)[/tex]
you will get
[tex]\left(\frac{2a+0}{2},\:\frac{0+2b}{2}\right)[/tex]
which is