Finding the magnitude of forces acting on a body

Two forces Fa and Fb are applied to an object whose mass is 8.0kg. Fa is the larger force. When both forces point due east, the objects acce. has a magnitude of .50m/s.s. However, when Fa points due east and Fb points due west, the acc is 0.40m/s.s. Find a) magnitude of Fa and b) the magnitude of Fb.

I tried it and came up with a) as 4N, but the answer is 3.6N. I did m x a=F

I don't have a clue how to get 0.40 N for the answer of b).

Can someone help me get started on this please.

I looked thru the little notes I have, which do not help and going thru my book, has nothing like this.

Thanks.

Joanne(Itwasntme)