How much Force does a brick with mass 4.5 kilograms exert on a window if it has an acceleration of 2.0 m/s^2?

Force F = m*a = 4.5*2 = 9.0 N .....but the question is a bit unclear 

This isn't the same as throwing a brick though a window because the brick is ONLY accelerating when force is applied to it.  Once a thrown brick is released, it is only accelerating DOWN at 9.8m/s². Its horizontal velocity is constant. 

F = m*a = 4.5*2 = 9N