Would a precipitate be observed if equal volumes of a 0.040 m agno3 solution and a 0.030 m nano2 solution are mixed?

When we have equals volumes of AgNO3 & NaNO2 so we can assume that the total volume = 1 L

so we need to new concentrations of:

[Ag+] = 0.04/2 = 0.02 M

and [NO2-] = 0.03 /2 = 0.015 M

So we need to get Q expression to compare it with the KsP value 

when Q = [Ag+][NO2-]

by substitution:

Q = 0.02 * 0.015 = 0.0003 

∴Q = 3 x 10^-4 by comparing this value with Ksp = 6 x 10^-4 (missing in your queston)

∴Q<Ksp 

and when Q < Ksp this means that, this is an unsaturated solution and there is no precipitate will be observed.

Putting salt in vinegar water