We know that the net force is the sum of the forces acting on the body then we need to add them; to do this we need to be careful and we have to remember that the forces are vectors, this means that we need to account for the directions in the sum.
For this problem we are going to assume the right direction is positive while the left is negative, then the net force is:
Therefore the net force on the crate is zero and since the crate was initially at rest this means that it will remain at rest, this comes from Newton's first law.