Which net ionic equation describes the reaction between k3po4(aq) when it is mixed with al(no3)3(aq)?

K₃PO₄(aq) reacts with Al(NO₃)₃(aq) and form AlPO₄(s) and KNO₃(aq) as the products. 

Before writing net ionic reaction, following steps should be followed.

Step 1 : Write the balanced chemical equation for the reaction and indicate the state of compound as aq,s,g or l.
      K₃PO₄(aq) + Al(NO₃)₃(aq) → AlPO₄(s) + 3KNO₃(aq) 

Step 2 : Identify the ionic species which dissolve in water to form ions.
K₃PO₄(aq) → 3K⁺(aq) + PO₄⁻(aq) 
Al(NO₃)₃(aq) → Al³⁺(aq) + 3NO₃⁻(aq) 
KNO₃(aq) → K⁺(aq) + NO₃⁻(aq)

Step 3 : Write the equation again by using ions.
3K⁺(aq) + PO₄⁻(aq) + Al³⁺(aq) + 3NO₃⁻(aq) → AlPO₄(s) +3K⁺(aq) + 3NO₃⁻(aq)

Step 4 : Identify the similar species in both side of the equation and cut off them.
3K⁺(aq) and 3NO₃⁻(aq) present in both sides. Hence, those ions can be cut off.

Step 5 : Get the final ionic equation.
The net ionic equation for the given reaction is 
      Al³⁺(aq) + PO₄⁻(aq) → AlPO₄(s) 

Yes you are correct use the scientific method