From the question Kp=4
When the reaction reaches equilibrium the partial pressure of the products become equal.
NH2COONH4 is solid so it's partial pressure can be neglected.
Kp= (2p)2p
4=4p3
p3=1
So p=1
Partial pressure for NH3=2p=2×1=2
Partial pressure for CO2=p=1
Therefore total pressure is=2+1=3