All forces on the bullets cancel so that the net force on a bullet is zero, which means the bullet has zero acceleration and is in a state known as

The acceleration is defined as the rate of change of velocity.
So, if the acceleration is zero, this means that the rate of change of velocity is zero, which also means that the body is moving with constant velocity.
Since we are given that the net forces acting on the body is zero, this means that the body is at equilibrium

Based on this:
All forces on the bullets cancel so that the net force on a bullet is zero, which means the bullet has zero acceleration and is in a state known as equilibrium.

Note that if this constant velocity is equal to zero, then the body would be at rest (not moving)

Wavelength divides in half and speed stays the same.